Arp et al 2015 BG

Arp et al 2015 BG
Biogeosciences, 12, 29–47, 2015
www.biogeosciences.net/12/29/2015/
doi:10.5194/bg-12-29-2015
© Author(s) 2015. CC Attribution 3.0 License.
Distribution and biophysical processes of beaded streams in Arctic
permafrost landscapes
C. D. Arp1 , M. S. Whitman2 , B. M. Jones3 , G. Grosse4 , B. V. Gaglioti1,3 , and K. C. Heim5
1 Water
and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, AK 99775, USA
Field Office, Bureau of Land Management, Fairbanks, AK 99709, USA
3 Alaska Science Center, U.S. Geological Survey, Anchorage, AK 99508, USA
4 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Potsdam, Germany
5 School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK 99775, USA
2 Arctic
Correspondence to: C. D. Arp ([email protected])
Received: 19 June 2014 – Published in Biogeosciences Discuss.: 24 July 2014
Revised: 14 November 2014 – Accepted: 26 November 2014 – Published: 6 January 2015
Abstract. Beaded streams are widespread in permafrost regions and are considered a common thermokarst landform.
However, little is known about their distribution, how and
under what conditions they form, and how their intriguing
morphology translates to ecosystem functions and habitat.
Here we report on a circum-Arctic survey of beaded streams
and a watershed-scale analysis in northern Alaska using remote sensing and field studies. We mapped over 400 channel
networks with beaded morphology throughout the continuous permafrost zone of northern Alaska, Canada, and Russia and found the highest abundance associated with medium
to high ground-ice content permafrost in moderately sloping
terrain. In one Arctic coastal plain watershed, beaded streams
accounted for half of the drainage density, occurring primarily as low-order channels initiating from lakes and drained
lake basins. Beaded streams predictably transition to alluvial channels with increasing drainage area and decreasing
channel slope, although this transition is modified by local
controls on water and sediment delivery. The comparisons of
one beaded channel using repeat photography between 1948
and 2013 indicate a relatively stable landform, and 14 C dating of basal sediments suggest channel formation may be
as early as the Pleistocene–Holocene transition. Contemporary processes, such as deep snow accumulation in riparian
zones, effectively insulate channel ice and allows for perennial liquid water below most beaded stream pools. Because
of this, mean annual temperatures in pool beds are greater
than 2 ◦ C, leading to the development of perennial thaw bulbs
or taliks underlying these thermokarst features that range
from 0.7 to 1.6 m. In the summer, some pools thermally stratify, which reduces permafrost thaw and maintains cold-water
habitats. Snowmelt-generated peak flows decrease rapidly by
two or more orders of magnitude to summer low flows with
slow reach-scale velocity distributions ranging from 0.01 to
0.1 m s−1 , yet channel runs still move water rapidly between
pools. The repeating spatial pattern associated with beaded
stream morphology and hydrological dynamics may provide abundant and optimal foraging habitat for fish. Beaded
streams may create important ecosystem functions and habitat in many permafrost landscapes and their distribution and
dynamics are only beginning to be recognized in Arctic research.
1
Introduction
Channels with regularly spaced deep and elliptical pools connected by narrow runs are a common form of many streams
that drain Arctic permafrost foothills and lowlands. These
channels are often referred to as “beaded” streams because
during summer low flows, pools appear as beads-on-a-string
of runs (Oswood et al., 1989). Beaded streams are generally treated in scientific textbooks on permafrost (e.g., Davis,
2001), hydrology (e.g., Woo, 2012), and aquatic ecology
(e.g., McKnight et al., 2008), yet to our knowledge field investigations of these systems has been limited to Imnaviat
Creek in northern Alaska (e.g., Oswood et al., 1989) and the
Yamal Peninsula in Siberia (Tarbeeva and Surkov, 2013).
Published by Copernicus Publications on behalf of the European Geosciences Union.
30
C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Table 1. Summary of a circum-Arctic inventory of beaded stream networks in the zone of continuous permafrost based on a survey of
high-resolution (< 5 m, summer) imagery available in Google Earth™ during 2012–2013. The relative proportion of high-resolution imagery
available in each region was used to estimate the total number of stream networks and drainage density assuming an average network length
of 10 km.
Region
Northern Canada
Northern Alaska (USA)
Northern Russia
Area
(km2 )
Percentage of area with
high-resolution
imagery (snow-free)
Identified
stream networks
Estimated
stream networks
Estimated
drainage density
(km km−2 )
2 347 072
185 907
2 123 067
9
80
11
22
275
148
244
344
1346
0.001
0.019
0.006
Figure 1. Examples of beaded stream networks located by scanning high-resolution (< 5 m) imagery available in Google Earth
in (a) Russia (Anabar River watershed), (b) USA (near Nuiqsut,
Alaska), and (c) Canada (Tuktoyaktuk Peninsula).
Our understanding of the physical and chemical character of beaded streams mainly comes from Imnavait Creek in
the Arctic foothills of Alaska (Oswood et al., 1989). Subsequent studies of this and adjacent systems suggest how
beaded morphology functions in permafrost thaw (Brosten
et al., 2006), hydrologic storage and hyporheic exchange
(Merck et al., 2012; Zarnetske et al., 2007), and thermal
regimes (Merck and Neilson, 2012). Thermal stratification
in pools up to 2 m deep often occurs in beaded channels during summer low flows (Oswood et al., 1989) and this may
play a role in permafrost thaw, hydrologic transport, and nutrient processing as the Arctic climate changes (Zarnetske et
al., 2008; Merck and Neilson, 2012). In the winter, foothill
streams freeze solid (Best et al., 2005) such that bed sediments thaw slowly and to a limited depth compared to adjacent alluvial channels (Brosten et al., 2006; Zarnetske et
Biogeosciences, 12, 29–47, 2015
al., 2007). Winter analysis of multiple aquatic habitats on
the Arctic coastal plain (ACP), however, shows that beaded
streams can maintain liquid water under ice and potentially
develop perennially thawed sediments (Jones et al., 2013).
These physical regimes of water and energy flow in Arctic streams, coupled with channel morphology and drainage
network organization likely also dictate how these ecosystems function as aquatic habitat (Craig and McCart, 1975).
Hydrographic analysis of the Fish Creek watershed on the
ACP show that beaded streams form the dominant connections between larger river systems and abundant thermokarst
lakes, thus influencing both hydrology and the movement of
aquatic organisms between habitats (Arp et al., 2012b).
Beaded streams are thought to be a common Arctic
thermokarst landform and occur mainly in association with
ice-wedge networks of polygonized tundra (Pewé, 1966).
The formation of channel drainage in these streams occurs along ice-wedge troughs with mature drainage channels resulting in complete degradation of ice wedges by thermal erosion (Lachenbruch, 1966). Classification of Arctic
streams place beaded channels within the tundra class as
compared to springs and mountain classes (Craig and McCart, 1975). In foothill watersheds, beaded streams are typically fed by linear hillslope water tracks (McNamara et
al., 1999), while on the ACP these channels initiate mainly
from thermokarst lakes and drained thermokarst lake basins
(DTLBs) (Arp et al., 2012b; Whitman et al., 2011). Based
on existing research, it is uncertain whether high densities
of beaded streams exist beyond this long-standing focal site
(Imnavait Creek/Toolik Lake) and this more recent studied
watershed (Fish Creek). Newly published work from Russian
permafrost zones is also expanding our knowledge of beaded
stream distribution (Tarbeeva and Surkov, 2013). Still, an understanding of their formative processes and the broader watershed functions they provide are currently lacking.
Knowing where beaded streams occur in permafrost landscapes and how these fluvial forms are organized within
drainage networks will help advance our understanding of
their broader role in watershed, ecosystem, and biological
functions across the Arctic. Such analyses will also help
in predicting changes in these thermokarst fluvial systems
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
31
Figure 2. The distribution of beaded streams located using Google Earth and from aerial surveys across the North Slope of Alaska in relation
to permafrost ice content (Jorgenson et al., 2008) and the Pleistocene glacial maximum (Manley and Kaufman, 2002). The locations of the
Fish Creek watershed (focus area of this study) and Imnavait Creek (focus area of the majority of pervious work on beaded streams) are
indicated.
2
2.1
Figure 3. The drainage network of Fish Creek watershed (location
shown in Fig. 2) showing all beaded stream networks that were delineated from 2.5 m CIR photography. River systems and individual
beaded stream catchments where more detailed field and geospatial
studies were conducted for this study are indicated.
with respect to climate and land-use changes and corresponding permafrost responses and hydrologic feedbacks. In this
study, we (1) describe the distribution of beaded streams
from circum-Arctic to regional scales, (2) explore whether
the distribution and variation in beaded morphology helps explain physical functioning, the evolution of beaded streams,
and their responsiveness to external drivers, and (3) highlight the important role that these ecosystems serve in aquatic
habitat. This work expands our understanding of beaded
streams beyond the foothill regions of Arctic Alaska where
most of all previous work has been completed, both in terms
of fundamental aspects of permafrost and fluvial processes
as well as aspects relevant to fish and other aquatic biota.
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Methods
Study areas, distribution surveys, and classification
The distribution and abundance of beaded streams were determined by using a nested survey design and a range of
survey methods. These nested domains ranged from a (1)
circum-Arctic assessment confined to the zone of continuous permafrost using imagery in Google Earth (GE) (Table 1,
Fig. 1), (2) aerial transects across landscape gradients on the
North Slope of Alaska (Fig. 2), and (3) a census of the Fish
Creek watershed (4700 km2 ) using high-resolution photography (Fig. 3). We also conducted field studies throughout this
watershed and used data from an ongoing monitoring network at several streams in the lower portion of the watershed
to characterize biophysical processes and habitat.
The circum-Arctic survey utilized imagery available in GE
to identify channels with beaded morphology. This analysis focused on the continuous permafrost zone north of 66◦
latitude. We utilized the historical image browser function
in GE to access the highest resolution imagery (< 5 m) possible for a given region. This analysis focused on portions
of Alaska (USA), Siberia (Russia), and northern Canada totaling approximately 4.5 million km2 . We found that most
channels with beaded morphology could be identified when
scanning images at a 1 : 6000 scale when the imagery had
a resolution of 5 m or finer and was mostly snow-free. The
availability of high-resolution, snow-free imagery in Alaska
was quite good, covering 80 % of the continuous permafrost
zone surveyed. In Russia and Canada, the availability of
such imagery was much lower, 11 and 9 %, respectively, as
of 2013 (Table 1). Prospective beaded channels recognized
while scanning were inspected more closely (finer scale) to
verify their form and the course was marked as the furthest
downstream network point of the continuous beaded channel.
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 4. Oblique photographs showing typical pool-run morphology (a) and examples of beaded channel forms (b–e) compared to
alluvial channel (f) morphology.
Surface elevation, latitude, and classes of permafrost ground
ice were attributed to each point using thematic data sets
for pan-Arctic (Brown et al., 1998) and Alaska-focused permafrost and ground ice distribution (Jorgenson et al., 2008)
and surface elevation. In order to compare among regions
with differing extents of sufficient imagery, we extrapolated
the number of surveyed streams based on the proportion of
high-resolution imagery available to estimate the total number of beaded stream networks in the circum-Arctic continuous permafrost zone (Table 1). We additionally estimated
drainage density of beaded channels based on assuming an
average network length of 10 km, which results in only a
broad regional average and definitely varies considerably on
finer scales.
Regionally (Alaska North Slope) focused aerial surveys in
a Cessna 185 were flown on 10 July 2011 on a clear day along
three transects. One 270 km transect was from the Brooks
Range divide north to the Colville River delta, which moves
from glaciated terrain in the upper foothills to vast areas
north of the Pleistocene glacial maximum (Fig. 3). Another
transect was 130 km from Prudhoe Bay to the lower Fish
Creek watershed on the ACP, and a third transect spanned
36 km of land area from Fish Creek to the lands north of
Teshekpuk Lake representing an inner to outer ACP gradient.
During the transect flights at approximately 150 m elevation,
one observer had a sufficient view of approximately 500 m
land surface to one side of the plane, thus covering approxiBiogeosciences, 12, 29–47, 2015
mately 220 km2 of land surface in these surveys. During the
flight each stream observed was marked with a GPS, photographed, and later these photographs were inspected to determine which streams could be classified as having beaded
morphology.
The watershed census of beaded streams was conducted
in the Fish Creek watershed as part of a broader effort to
map, classify, and understand watershed hydrography and
its role in watershed runoff processes (Arp et al., 2012b).
The Fish Creek watershed is located in the northeastern portion of the National Petroleum Reserve – Alaska (NPR-A)
on the ACP (Fig. 3). Surface deposits grade from marinealluvial silt with some pebbly substrates in the east to inactive eolian sand dune fields in the west (Carter, 1981; Carter
and Galloway, 2005). The sand-bedded alluvial rivers, Fish
Creek (Uulutuuq, Iñupiat name) and its tributary Judy Creek
(Iqalliqpiq), drain this area and form a delta in the Beaufort Sea just west of the Colville River delta. Both rivers
begin as beaded streams, Judy in a narrow arm extending
into the foothills and Fish in the sand sea. The Ublutuoch
River (Tingmiaqsiuqvik) also starts as a beaded stream, but
maintains this morphology for a longer distance before becoming a gravel-bedded alluvial channel near its confluence
with Fish Creek (Fig. 3). All perennial channels in the Fish
Creek watershed were delineated from 2002 mid-July color
infrared (CIR) photography (2.5 m resolution) in a GIS environment. Streams with beaded morphology were quantified
according pool density and size (measured as width perpendicular to the direction of flow) and valley gradient from a
5 m interferometric synthetic aperture radar (IfSAR) digital
elevation model (DEM) at a segment scale, typically a 1–
3 km length that was representative of individual drainage
networks. These segments were also placed into four classes
according to predominant pool (channel bead) shape and
connectivity to runs: (1) elliptical (round) pools separated by
distinct connecting runs (Fig. 4b), (2) coalesced pools (elliptical pools merged together) without distinct connecting runs
(Fig. 4c), (3) large irregularly shaped pools often connected
by long runs (Fig. 4d), and (4) connected thaw pits in degrading polygonized tundra connected by perennial or ephemeral
streams (Fig. 4e). We used this classification to help evaluate if pool form of beaded morphology was correlated with
landscape position within the watershed and permafrost icecontent or other thermokarst landforms (e.g., thermokarst
lakes and DTLBs). We visited approximately 20 % of these
stream channels in the Fish Creek watershed during late July
2011 to verify beaded morphology and classification and to
collect additional field measurements, as described below in
the next section.
2.2
Geospatial and field measurements
A subset of stream channels mapped and classified in the
Fish Creek watershed (Arp et al., 2012b) were used for
detailed geomorphic and hydrologic analysis in this study.
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Specifically, we targeted a set of each channel class representing beaded streams and alluvial channels (Fig. 4), as
well as points of channel initiation. During field visits, we
measured stream discharge using the velocity–area method.
Along stream reaches equaling 20 or more channel widths
(typically 100–300 m), we surveyed the water surface elevation at 5–7 points with an engineer’s level, stadia rod, and
tape to measure the channel slope. At the same time, channel
cross sections that bisected pools were surveyed at 2–3 locations to measure pool geometry as well as the incised zone
surrounding the channel (gulch) indicated by riparian vegetation and form.
In order to better understand controls on beaded stream
morphology, we conducted similar surveys in the field, and
from geospatial data (CIR photography and DEMs) along a
longitudinal gradient of Fish Creek and the Ublutuoch River
from their headwaters downstream (Fig. 3). For each fluvial
system, at least three reaches were studied in the field where
the channel had distinctly beaded form and three reaches
were studied downstream where the channel had transitioned
to an alluvial form. Additional locations were later selected
to better refine this transition including identification of sediment sinks (flow-through lakes) or clear-water inputs (lakefed tributaries) relative to potential sediment sources including contact points with hillslopes and sand dunes, and tributaries originating from DTLBs or upland tundra. Such local
controls on delivery of new water and sediment to channels
were expected to help explain changes in form downstream,
similar in concept to mountain drainage networks flowing
through lakes (Arp et al., 2007) and as hypothesized for Arctic drainage networks (Tarbeeva and Surkov, 2013). The total length of channels analyzed for the Fish Creek watershed
was about 135 km and the total length of channels analyzed
for the Ublutuoch River watershed was about 70 km.
2.3
Analysis of channel change and history
To better understand the evolution of beaded channels we
compared the position and morphology of one channel over
a 64 a period using high-resolution (1 : 24 000 scale) photography from 1948 (black and white, Naval Arctic Research
Laboratory, BW NARL) and 2013 (color-infrared at 25 cm
pixel size, Aerometric Inc) located in the Fish Creek watershed. This was done to examine the hypothesis that beaded
streams evolve in a manner similar to observed degradation
of ice-wedge intersections, but lacking channel connectivity.
The 1948 BW NARL photographs were acquired from the
University of Alaska Fairbanks GeoData Center and scanned
at 1200 dpi. The scanned images were georeferenced with 20
ground control points (primarily, stable ice-wedge intersections) to a light detection and ranging (lidar) data set (detailed
below) using a spline transformation and converted to a pixel
size of 0.5 m. The 2013 color photography was acquired, by
Aerometric Inc., on 4 September to complement airborne lidar data. Manual analysis of both data sets was conducted in
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33
black and white to avoid any bias that may have arisen due
to differences in film types and their separation by so many
years of time. Particular attention was given to any changes
in channel form (location and plan-view dimensions) relative to ambient polygonized tundra within a 100 m buffer of
the channel and the presence and dynamics of thaw pits. All
stream channels in both images were independently delineated manually and individual pools and ice-wedge intersections with pits marked with a central point. We tracked individual pools (beads) and thaw-pits from 1948 to 2013 and
also recorded those features that were observed in one time
period but not the other. The channel gulch/riparian corridor was also delineated for both periods, based primarily on
the darker (greener) signature of taller sedges, willows, and
dwarf birch and moister understory bryophyte communities.
In order to estimate the timing of pool initiation, longterm sedimentation rates, and the depositional environment
of pools, we collected sediment cores to analyze sediment
stratigraphy and estimate age–depth relationships using 14 C
dating. In April 2012, two overlapping cores were collected
from a large, deep pool in Crea Creek (Fig. 3) to a depth
of 75 cm (base of unfrozen talik) using a Russian peat corer.
Each core was photographed and subsampled at 5 cm increments with subsamples placed in Whirl-Pak™ bags. Here we
identified what appeared to be basal sediments where the
channel initiated, as indicated by an organic sediment layer
with fibrous terrestrial organic remains sitting above a homogenous and thick sand layer extending down into the base
of the talik. We sampled an individual twig from this basal
section for 14 C dating. Several moss and sedge samples were
also collected from above the basal layer in organic-rich,
sandy sediments, similar to organic-rich gyttja deposited in
lakes of the region, for dating as well. Another core was
collected from a pool in 2013 at nearby Blackfish Creek
(Fig. 3) and macrofossils were collected from above several
distinct sand horizons within the core. The plant macrofossils
were prepared for analysis with an acid-base treatment and
analyzed for 14 C content using standard acceleratory mass
spectrometry techniques at the NOSAMS (National Ocean
Sciences Accelerator Mass Spectrometry) facility at Woods
Hole Oceanographic Institute. All radiocarbon dates were
calibrated to calendar ages using the IntCal 13 curve (Reimer
et al., 2013) and are reported as the mean and 2σ ranges of
the calibrated ages.
2.4
Hydrologic monitoring and habitat analysis
As part of an ongoing monitoring program (Fish Creek Watershed Observatory; Whitman et al., 2011), streamflow, water temperature, and other water quality parameters have
been recorded at hourly intervals at five stream-lake systems
since 2008. These small catchments (Fig. 3) are being monitored by the Bureau of Land Management (BLM) Arctic
Field Office to collect baseline data prior to expected changes
in land use, primarily new oil development, and associated
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 5. The distribution of beaded stream channels throughout
the circum-Arctic in relation to latitude, elevation, and permafrost
ground-ice content. Stream networks were identified using imagery
in Google Earth in the zone of continuous permafrost where highresolution imagery was available. The location of streams in the
Fish Creek (focus area for this study) and Imnavait Creek (focus
area for majority of previous beaded stream research) watersheds
are indicated with yellow stars.
lake-water extraction for ice road construction and facility
operations in the NE NPR-A. Stream gauging was conducted
using autonomous pressure transducers (Onset U20-001-01)
anchored to pool beds, which were corrected to local atmospheric pressure to measure water height. Stream discharge
was measured using the velocity–area method with either
an ADCP (acoustic doppler current profiler; Flowtracker™ )
or electromagnetic (Hach™ ) velocity meter mounted to a
top-setting wading rod. Approximately 20 velocity measurements were made per cross section at increments spaced to
not exceed 10 % of total discharge. Typically we made 3–4
measurements near the snowmelt peak flow in early to midJune, 2–3 measurements during peak-flow recession in late
June or early July and 2–3 measurements again in late July
and late August. Rating curves were fit with a log or power
law equation to estimate continuous discharge during the icefree season; separate high-flow and low-flow rating curves
were often required. Based on temperature sensors placed
in channel runs and comparison with time-lapse cameras set
during several years, we assumed that streamflow ceased during October in most years.
We tested how contrasting beaded stream morphometry
and watershed features affected hydrologic residence times
and velocity distributions using tracer tests on two stream
reaches with contrasting morphology and flow regimes
(Fig. 6). At Crea and Blackfish creeks (Fig. 3), we identified 325 and 232 m reaches, respectively, starting and ending
at channel runs to ensure initial mixing and sampling of the
Biogeosciences, 12, 29–47, 2015
Figure 6. Morphological characteristics of beaded streams compared according to pool (bead) density, size, and shape classes (examples shown in Fig. 4 and locations shown in Fig. 3) at the segment
scale (1–3 km channel length) in the Fish Creek watershed.
advective flow. Rhodamine WT (RWT), a pink fluorescent
dye, was used as a water tracer because it can be detected
at low concentrations and only small quantities are required
to reach target concentrations, which is an important practical consideration for remote field sites. RWT has low biological reactivity, yet does sorb to organic matter and begins photodegrading after several days of sunlight exposure
at low concentrations (Vasudevan et al., 2001). Thus, RWT
is not truly conservative,however, is widely used to characterize channel hydraulics and transient storage processes, including previous work in Arctic beaded streams (Zarnetske
et al., 2007). Based on targeted downstream peak concentrations of 30 ppb (parts per billion), we made pulse additions of
RWT at reach heads and monitored concentration at the reach
bottom using a YSI 6600-V2 water quality sonde with a
RWT probe. This experiment typically lasted a day or longer
to account for all tracer moving through the system. RWT
tracer data were then fit with the model One-dimensional
Transport model with In-channel Storage and Parameterization (OTIS-P) to estimate advective channel area (A), storage
zone area (AS ), dispersion (D), and the storage exchange
coefficient (α) (Runkel, 2000). The percentage of RWT recovery averaged 81 % with an average sorption coefficient
(λ) of 1 × 10−5 used to account for this loss downstream.
A tracer breakthrough curve data was plotted as cumulative
solute recovered downstream and converted to velocity distribution by dividing reach length by travel time. RWT injections were conducted at both Crea and Blackfish creeks in
mid-June near peak flows, in early July (late peak-flow recession), and late August (low summer baseflow).
Stream thermal regimes were quantified using the same
pressure transducers anchored to pool beds that also record
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
temperature, along with thermistors (Onset U12-015) near
the surface of pools (30 cm below) and in channel runs of
each beaded stream; all recording at hourly intervals. These
paired temperature measurements were used to assess thermal regimes and timing and extent of stratification in pools
assuming that a ratio of surface temperature to bed temperature > 1.1 indicated stratification. Using this system, one
pool and corresponding channel run have been monitored
among five streams year-round from 2009–2013 (Fig. 3). To
assess variability in thermal regimes and particularly stratification within stream systems, we selected an additional three
pools of varying depth and area in both Crea and Blackfish
creeks (Fig. 3) in 2012 and instrumented these with additional bed and surface thermistors. These were retrieved and
downloaded in late August 2013.
During the late winters (March and April) of 2010–2013,
we visited several of these same beaded stream reaches concurrent with lake-ice, snow, and water chemistry surveys.
When opportunities existed, we measured snow depth either with a 3 m avalanche probe or by digging a pit, or both,
above frozen pools located with a GPS. Holes were augered
through the ice and ice thickness and below-ice water depth
was measured using an ice-thickness gauge (Kovacs Enterprises LCC™ ). We also measured the depth of thawed sediment (talik) using multiple 1.2 m threaded stainless steel rods
fitted with a blunt tip and driven with a slide-hammer to the
depth of refusal (typically 10–20 pounds with no downward
movement). When possible, these late winter surveys were
done repeatedly at the same pools including measurements
of dissolved oxygen, specific conductance, and pH to assess
the quality of overwintering fish habitat.
3
3.1
Results and discussion
Beaded stream distribution
Using available high-resolution imagery in GE across the
circum-Arctic, we found 445 individual channel networks located in northern Alaska, Russia, and Canada with beaded
morphology (Table 1). This survey was restricted to land areas north of 66◦ latitude, which was mainly in the zone of
continuous permafrost, though two streams identified were
within areas classified as discontinuous and three within
areas classified as sporadic permafrost. The availability
of high-resolution snow-free imagery strongly reduced the
number identifiable channels in Siberia and Canada. Extrapolations to the entire region of continuous permafrost based
on the area we could accurately survey, suggests greater than
1900 individual beaded stream networks with 13 % in northern Canada, 18 % in Alaska, and 69 % in northern Russia
(Table 1). The density of beaded streams in Alaska was estimated to be about 3 times higher than in Russia and 19
times higher than Canada, likely related to its small but wide
unglaciated, ice-rich permafrost coastal plain of the Alaska
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35
North Slope relative to abundant mountain and foothills terrain of much of northern Russia and the expansive Laurentian
Shield covering much of northern Canada.
In Russia, 148 beaded streams were identified and clustered mainly in several different locations. From east to west
these included coastal plains along the Chukchi Sea, lakerich valley bottoms west of the Kolyma Delta, mountainous
headwaters of the Yakutia region, higher elevations of the
Yamal Peninsula, and very high densities in the foothills of
the Anabar River watershed (Fig. 1a). Recent field studies
were completed on beaded streams on the Yamal Peninsula
and these researchers also remotely identified channels with
beaded morphology in other Russian taiga and steppe terrains using Google Earth (Tarbeeva and Surkov, 2013). Comparatively fewer beaded streams were identified across the
Canadian Arctic (22 total) (Table 1). This is likely related to
regional geology associated with the dominance of exposed
bedrock and thin sediment cover and lack of ice wedges on
the Canadian Laurentian Shield. From west to east, small
clusters of beaded streams were found on the coastal plain
east of Herschel Island and south of the Mackenzie River
delta, the lake-rich Tuktoyaktuk Peninsula (Fig. 1c), the
coastal plain around the Coronation Gulf and village of
Kugluktuk, and the Banks Peninsula within Bathurst Inlet,
where high-resolution imagery was available during this GE
survey.
Because of greater availability of high-resolution imagery,
over 60 % of the beaded streams we located were in Alaska
even though this was a much smaller area surveyed (Table 1).
The southernmost beaded streams in Alaska were found on
the coastal plain of the Seward Peninsula and between Kivalina and Point Hope with an additional cluster higher in the
Noatak River valley (Fig. 2). On the North Slope of Alaska,
beaded streams were dense and more evenly distributed in
the western foothills and along the Chukchi coastal plain.
Lower densities of beaded streams were found in the central sand sea region and only a few beaded channels were
found on the outer coastal plain of the Barrow Peninsula
and north of Teshekpuk Lake. This lack of channels with
beaded morphology on the outer coastal plain is perhaps unexpected, given the ubiquitous presence of ice-wedge polygons in which beaded drainage forms. We have observed
however that most channels in this region tend to take a plane
bed form without alluvial features, which may relate to very
high pore-ice content that in addition to wedge-ice makes
soils in this region extremely ice-rich, often exceeding 80 %
by volume (Brown, 1968, Kanevskiy et al., 2013). The outer
coastal plain is also extremely flat with very low drainage
densities and very high coverage of thermokarst lakes and
DTLBs (Hinkel et al., 2005), such that all fluvial systems
are in low abundance and the ones present are strongly lakeaffected. On the inner coastal plain and foothills, channels
likely develop along moderately sloping terrain with varying densities of ice wedges, but otherwise low pore-ice content. Thus, bead morphology likely develops as ice-wedge
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36
C. D. Arp et al.: Distribution and biophysical processes of beaded streams
networks thermally erode, yet the expansion of pools and
runs is confined to the original ice-wedge casts likely because ice-poor permafrost is more resistant to thermokarst
erosion. High densities of beaded streams were also found
throughout the Kuparuk River watershed from the foothills
to the coastal plain and on the narrower coastal plain east of
the Sagavanirtok River to Barter Island (Fig. 2).
Looking at the full set of beaded streams in relation to
the ground-ice content of permafrost shows that 50 % were
found on high ground-ice content permafrost, 32 % on moderately high ground-ice content permafrost, and 18 % on
low ground-ice content permafrost (Fig. 5). Regions with
high ground-ice content were typically associated with either epigenetic permafrost along the coastal region and
syngenetic yedoma permafrost in the foothills region. Approximately 50 % of all beaded streams were found below
60 m a.s.l. (above sea level) elevation and 90 % were found
below 210 m a.s.l. elevation (Fig. 5). Seven beaded streams
were discovered above 500 m a.s.l. These were found in both
Alaska and Russia. Our survey did not identify the even
higher-elevation Imnavait Creek, 861 m elevation (Figs. 2,
5), since the only high-resolution GE imagery for this area
was acquired during winter snow cover when beaded morphology could not be observed, demonstrating the limitations
associated with this identification approach. However, such
snow-covered scenes were relatively rare in most imagery
we used. Imnavait Creek, along with 12 beaded streams that
were identified in our inventory, occurs above the Pleistocene
glacial maximum (Fig. 2) indicating that streams with beaded
morphology can readily form in glaciated terrain.
In our aerial surveys across the Alaskan North Slope,
we located 43 beaded streams from three transects covering
436 km of flight lines or approximately 220 km2 , suggesting
a density of 0.20 streams per square kilometer or a drainage
density of roughly 0.10 km km−2 . Comparing transect lines
to landscape classification of permafrost ground-ice content
shows that these surveys covered (29 %) low, (59 %) moderate, and (12 %) high categories (Fig. 2). However, of the recognized beaded streams along these courses, a much higher
proportion was associated with moderate ice-rich permafrost
(76 %). Only three streams occurred on high ground-ice content permafrost, two on very flat outer coastal plain areas
with glaciomarine sediments, and one in yedoma deposits of
the foothills (Fig. 2). The majority of stream channels on the
outer coastal plain, with very low drainage densities, would
be generally classified as plane bed (Montgomery and Buffington, 1997) or F5-6 from Rosgen’s classification (Rosgen,
1994), and would have also been termed lacustrine channels
(Arp et al., 2012b) because they are nearly all mostly fed by
lakes. Still, polygonized tundra tends to be more pronounced
and uniform in this region, and so a general lack of channels
with beaded morphology was unexpected.
Beaded streams in the Fish Creek watershed range in elevation from 6 to 125 m and the full range of permafrost,
ground-ice contents (Jorgenson et al., 2008). We inventoBiogeosciences, 12, 29–47, 2015
ried 126 beaded streams as individual catchments or drainage
networks within this 4700 km2 watershed located on the inner Arctic coastal plain of northern Alaska (Fig. 3). Based
on previous analysis of lakes, streams, and river channels
here (Arp et al., 2012b), beaded streams represent 1168 km
of channel length or 47 % of the entire fluvial system. The
equivalent drainage density of beaded stream channels is
0.25 km km−2 . Estimated drainage densities for the broader
regions surveyed with GE were far lower compared to this
watershed (Table 1).
Since the majority of beaded streams on the ACP initiate
as first-order channels below thermokarst lakes or DTLBs
(Arp et al., 2012b), their distribution throughout the Fish
Creek watershed is linked to lake distribution (Fig. 3). The
exception to this pattern is in the headwaters of Judy Creek,
which form a narrow arm extending into eolian silt deposits
with bedrock outcrops. In this area, lake densities are low
and many streams initiate as colluvial channels (Arp et al.,
2012b), which then transition to beaded morphology downstream, similar to patterns reported for the higher-elevation
foothills of the Kuparuk watershed (McNamara et al., 1999).
An example of this drainage pattern is also evident in Fig. 1a.
Thirteen percent of all beaded streams in the Fish Creek watershed are located within this region of ice-rich eolian loess.
Relatively lower densities of beaded streams occur in the eolian sand sea regions (western half of Fish Creek watershed)
where permafrost is classified as having low ground-ice content (Fig. 2) and where most lakes formed between relict
dunes (Jorgenson and Shur, 2007) and are up to 20 m deep
(Arp et al., 2012b). The highest densities of beaded streams
occur in the lower Fish Creek watershed where surface geology is dominated by alluvial and marine silts and sands with
some pebbly deposits and permafrost is moderately ice-rich
(Carter and Galloway, 2005). Our results suggest some variation in beaded stream distribution within the inner coastal
plain, particularly with lower densities associated with eolian
and alluvial sand deposits and higher densities on marine and
loess silt deposits. However, we still find that beaded streams
are often the dominant form of low-order channels throughout a wide range of permafrost terrain on the Alaska North
Slope and this is likely the case in much of northern Russia
as well (Tarbeeva and Surkov, 2013).
3.2
Morphology in relation to landscape and watershed
positions
Since abundant large, deep pools are the defining characteristic of streams with beaded morphology, we initially classified and quantified these channels according to pool (bead)
morphology and density (Fig. 6). On a reach scale (hundreds
of meters) or segment scale (up to several kilometers between tributary junctions), pool density, form, and size was
often distinct. However, on a more extensive drainage network scale, which is the scale we used for classification,
pool density varied to a greater extent. Counts of pools from
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
high-resolution CIR photography showed densities ranging
from 2 pools per 100 m of channel up to 10 per 100 m
(Fig. 6). Lachenbruch (1966) suggested that polygon spacings range from 5 to 50 m based on variation in ground
strength and the width of stress relief zones, which approximately matches the range of beaded densities reported here.
This indicates that local controls, such as size, pattern, and
form (i.e., low- and high-centered polygons) of tundra or
broader-scale thermokarst landforms such as DTLBs (Frohn
et al., 2005; Hinkel et al., 2005), may be the main cause of
such variability in channel morphology.
Of the 126 individual beaded channel networks in the Fish
Creek watershed, 40 % were classified as elliptical with distinct connecting runs, 17 % had mostly coalesced pools and
short or non-existent runs, 34 % had predominantly irregularly shaped pools, and the remaining 8 % were classified
as connected thaw pits (Figs. 4, 6). The majority of beaded
channels are shown to initiate from either lakes and DTLBs
(Arp et al., 2012b) and these took a wide range of pool
forms downstream. In the Fish Creek watershed, most channels with small elliptical pools were located in the higher
elevation areas associated with eolian sand and loess deposits compared to lower elevation marine sand and silt deposits. Whether this pattern relates to size and form of icewedge networks that develop in sandy soils or how eroding
sandy soils moderate expansion by infilling pools or interactions with vegetation deserves further consideration. The
other channel classes were more evenly distributed throughout the watershed and by surficial geology.
Comparing channels of the entire watershed by individual slope and drainage area helps us to understand how the
larger drainage network is organized from channel initiation points (channel heads) to larger alluvial sand-bedded
channels (Fig. 7). This slope–area relationship is consistent with patterns more universally observed across a wide
range of drainage networks (Montgomery and Buffington, 1997; Montgomery and Dietrich, 1989; Whiting and
Bradley, 1993). In the Fish Creek watershed, channels initiating from hillslopes are steepest with slopes averaging 2 %
and with drainage areas < 1 km2 . Channels initiating from
lakes, which all form beaded streams, had average slopes of
0.4 % and drainage areas > 1 km2 (Fig. 7). Channel initiation thresholds reported for the foothill’s beaded stream Imnavait Creek are 0.02 km2 (McNamara et al. 1999) – roughly
1 and 2 orders of magnitude smaller than hillslope- and lakeinitiated channels, respectively, in this ACP watershed. Because beaded channels compose approximately half of the
drainage network in the Fish Creek watershed (Arp et al.,
2012b), they correspondingly have a wide range of drainage
areas (2–54 km2 ) and slopes (< 0.1–0.8 %) (Fig. 7). Analysis of beaded channels in Yakutia, Russia, show a narrower
range of drainage areas (3–10 km2 ) with slopes smaller than
0.2 % (Tarbeeva and Surkov, 2013). Alluvial channels form
the higher-order portions of most drainage networks and in
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37
Table 2. Comparison of beaded stream morphology and ambient thermokarst features between black and white photography acquired in 1948 and color infrared photography acquired in 2013 for
a 2.7 km segment of Crea Creek in the lower Fish Creek watershed.
Attribute compared
1948
2013
Pools
number
total area (m2 )
mean area (m2 )
number unique
132
7861
59.6
23
134
8334
62.2
25
221 802
82.1
241 247
89.4
74
55
66
47
Gulch/riparian zone
total area (m2 )
mean width (m)
Thaw pits
total number
number unique
the Fish Creek watershed they typically begin at drainage areas > 40 km2 and channel slopes smaller than 0.03 % (Fig. 7).
To better understand how beaded streams fit within fluvial
systems of the ACP and evaluate what controls their morphology, we selected two drainage networks for more detailed analysis of longitudinal channel dynamics from headwaters downstream (Fig. 8). Fish Creek has its headwaters
near the western divide of the watershed at 78 m a.s.l. It is
located entirely within the eolian sand sheet and initiates
from a deep depression lake (Fig. 3). This channel network
first flows through several more depression lakes and in between maintains a classic beaded morphology (Fig. 4a). Over
the next several kilometers, the channel cuts through both
vegetated and unvegetated sand dunes, which likely supply
coarse sediment. The channel also contacts steeper hillslopes
that could contribute sediment as well. This portion of the
channel appears transitional since reaches of beaded morphology are interspersed with more sinuous channels having point bars and meander cut banks (Fig. 8a). At kilometer
20 downstream, the channel steepens considerably below a
tributary fed by a DTLB and then cuts through two more
sand dunes, before taking a more even slope for the remaining 110 km with sand-bedded alluvial characteristics. Thus,
Fish Creek quickly transitions from beaded to alluvial morphology likely because of ample sediment supply associated
with the eolian sand landscape (Fig. 6a).
The other system we analyzed, the Ublutuoch River
(Fig. 3), begins at a lower elevation than Fish Creek,
58 m a.s.l., in the southern portion of the watershed at the
eastern margin of the eolian sand sheet. The channel initiates
from a large set of coalesced depression lakes, totaling about
5 km2 , seen as the flat profile in Fig. 8b. The first 12 km of
this stream are relatively steep with a regular density of pools
typical of beaded morphology. Several oxbow lakes occur
lower in this segment, indicative of channelmigration, but the
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38
C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 7. The organization of the major channel forms and channel
initiation points (heads) in the Fish Creek watershed are shown in
relation to drainage area and channel slope (measured from a 5 m
DEM).
Ublutuoch then flows through several more lakes, likely trapping all sediment and resetting the system to a beaded form
with a flatter slope. At kilometer 24 downstream, a tributary
from a large DTLB enters from the north and, at this point,
the channel starts taking a more sinuous form with oxbow
lakes and other floodplain features (Fig. 8b). We suggest that
this segment of stream from kilometer 24 to 56 is transitional
between beaded and alluvial morphology – a much longer
transition than was observed along the upper Fish Creek. Surrounding uplands here are entirely within the zone of marine
silt and sand without distinct sediment contributions from adjacent sand dunes. Near the end of the segment, the channel
becomes much more sinuous with oxbows and meander scars
becoming evident, yet regular pools (beads) persist. At kilometer 56, the stream contacts a distinctly higher hillslope that
we think supplies sediment to the channel and after which it
takes on a distinct alluvial form lacking any evenly spaced
beaded morphology (Fig. 8b). During the entire transitional
channel course, the slope is nearly constant at about 0.02–
0.04 %. It then flattens greatly to < 0.01 % over the last 5 km
and becomes quite deep (exceeding 5 m in some pools) and
very sinuous (2.3) with high, regular banks before its confluence with Fish Creek.
3.3
Channel change and formation
To evaluate the hypothesis that beaded streams form in icewedge networks and that pools progressively expand over
time, more detailed studies were conducted in one system,
Crea Creek, in the lower Fish Creek watershed (Fig. 3), to
look at decadal-scale changes and estimate its time of formation. Using remote sensing change detection over 64 a, we
found no changes in the channel position along this 2.7 km
Biogeosciences, 12, 29–47, 2015
Figure 8. Headwater to downstream patterns of a beaded stream
originating in the eolian sand deposits, Fish Creek (a), compared
with a beaded stream originating in alluvial–marine deposits, Ublutuoch River (b), showing changes in channel elevation and the density of pools and oxbow (meander-cutoff) lakes relative to sediment
sources and sinks.
segment (Fig. 9). The total number of pools in this segment
remained relatively stable, though tracking individual beads
showed that 18 % disappeared or could not be observed from
1948 to 2013 and a similar number of new pools (19 %) were
identified in 2013 that could not be observed in the 1948
imagery (Table 2). The majority of these were very small
(diameters < 4 m) and we think it is likely that changes in
vegetation or variation in water levels between images may
have obscured their detection. The mean pool size in 1948
was 60 m2 compared to 62 m2 in 2013, resulting in little net
change in total pool area over this period. Tracking the size
of individual pools found in both images showed that about
one-third shrank by more than 10 % surface area, about onethird expanded by more than 20 % surface area, and the remaining pools were essentially unchanged. Thus, our analysis suggests progressive expansion of these thermokarst landforms, yet the channel course appeared entirely unchanged
over this period. In comparison to other thermokarst landforms, thermokarst lakes in this region also progressively expand their lake basins, 0.10 m a−1 on average (Jorgenson and
Shur, 2007), but can drain catastrophically if a shoreline expands beyond a lower gradient or is breached by another lake
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 9. Comparison of two segments of the Crea Creek channel
in 1948 (a, c) and 2013 (b, d) showing that pools, the riparian gulch,
and adjacent thaw pits can be clearly observed in each image. The
location of a sediment core collected for 14 C dating is indicated
with a yellow triangle (location of Crea Creek is shown in Fig. 3).
or migrating river (Grosse et al., 2013). Alluvial channels on
the ACP are considered highly dynamic and often with very
high rates of bank erosion due to interactions with permafrost
such that major changes in channel course can occur over
short time periods (Scott, 1978). Our observations of a stable
course along Crea Creek over 64 a, along with an apparent
lack of beaded channels that appear abandoned on the ACP,
suggest long-term behavior more similar to bedrock channels
(Wohl, 2000). However, Tarbeeva and Surkov (2013) suggest
the beaded streams are transient features and become easily
filled with sediment from headwater thermokarst and other
hillslope erosive processes. We suggest that sediment delivery plays a role in how beaded streams transition to other
fluvial forms, but his typically operates at lower positions in
the watershed.
We also delineated the riparian zone or gulch of this
beaded stream, indicated in plan view by higher moisture
and the contrast between upland tussock tundra and vegetation composed of willows, tall sedges, and dwarf birch, to
see if other changes beyond the main channels were evident
(Table 2). Such changes could correspond to progressive subsidence of ice-rich permafrost by thermokarst degradation
or shrub expansion as has been noted throughout many areas of the Arctic (Sturm et al., 2001). Consistent with what
can be observed in the shorter reaches in Fig. 9, the overall change in riparian gulch width was slight, a 9 % increase
(Table 2). Analysis of repeat photography in this same area
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39
has shown a recent increase in degrading ice-wedge polygons
to form thaw pits (Jorgenson et al., 2006). We also recorded
and tracked thaw pits (ice-wedge junctions with ponded water) between the two images within a 100 m zone on either
side of the channel, but outside of the riparian gulch. This
showed a somewhat similar pattern as that found when tracking pools in the channel of Crea Creek. In total, we found 120
individual thaw pits or 1 pit per 2500 m2 , typically in clusters
associated with high-centered polygons. In 1948 we found
74 thaw pits, 55 of which were not observed in 2013, and in
2013 we found 66 thaw pits, 47 of which were not observed
in 1948 (Table 2). This suggests that thaw pits may progress
through a form of succession in which they degrade, collect
water, paludify and/or partly drain or dry, such that detection is obscured after several decades. This is a similar sequence to that demonstrated for denser networks of thaw pits
in polygonized tundra in adjacent upland areas of the Fish
Creek watershed (Jorgenson et al., 2006). We suggest that
beaded channels may evolve in a similar manner with most
pools gradually expanding and some contracting with changing vegetation. Such behavior seems particularly apparent in
viewing coalesced beads of some channels (Fig. 4c). Yet our
impression based on this photographic comparison and qualitative observation of other channels with repeat photography
is that channel courses and networks appear to behave more
like bedrock channels that are set in place and potentially
very old.
Analyzing the stratigraphy and geochronology of sediments in a large pool of Crea Creek may attest to the timing of stream channel formation and the depositional environment since initiation. A fibrous, organic-rich layer with
abundant terrestrial plant material separated the transition
from organic-poor, medium-grained sand to organic-rich,
silty sediment that is the uppermost unit – we interpreted this
layer as basal sediments that were dated to 9.0 (±40), and
13.6 (±215) ka cal BP (before present) (Fig. 10). The terrestrial macrofossils (shrub twigs) in this fibrous unit and the
two dates that span 4 ka suggest this layer may have been a
terrestrial soil that persisted for millennia on top of eolian
or alluvial sand deposits, but predated the initiation of the
beaded stream pool. Alternatively, this layer may represent
the depositional environment of an early stage of the beaded
stream pool where terrestrial vegetation was overhanging and
being deposited, and adjacent soils were being eroded by
ice-wedge degradation and supplying a range of reworked
material with different 14 C ages to be deposited onto this fibrous layer. Regardless, we interpret the 9.0 ka moss macrofossil sampled from the upper portion of the fibrous layer
to be a conservative upper limit age on the initiation of the
beaded stream pool. At this time, we do not know whether
the lower limit of this age estimate is near the 9.0 ka time
period, or represents the late Holocene. The large age gap
from 9.0 ka at 42 cm to ∼ 0.7 ka at 22 cm suggests that either
a water-level lowering event caused a hiatus of sedimentation through much of the Holocene, or that high-flow events
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 10. Diagram of generalized sediment core stratigraphy from
a large pool in Crea Creek (indicated in Fig. 9) collected in both
2012 and 2013 showing the location of macrofossil fragments collected for radiocarbon dating. The sharp transition from organicrich gyttja to medium sand is interpreted to be the base of the pool
at its time of formation.
Figure 11. Late winter profiles (March or April) of several pools
(beads) surveyed in multiple beaded streams from 2010 to 2013 (“?”
indicated that no measurement of thawed sediment depth was attempted). An example photograph from one pool surveyed in 2013
shows a 1.9 m tall person (G. Grosse) standing on the frozen pool
surface for scale.
or other processes eroded the sediment deposits representing
most of the Holocene (Fig. 10). However, there was no preserved wetland or terrestrial soil layer interrupting the gyttja
unit, which would have accompanied a water-lowering event.
The Crea beaded stream pool we examined appears to have
had episodic sedimentation during the Holocene that is periodically eroded by either high-flow events or ice scouring.
The stratigraphy and 14 C dates from a core in a deep
pool in Blackfish Creek also suggest unconformities in sedimentation of beaded stream pools. The Blackfish pool had
sandy organic-rich gyttja with several 3–6 cm bands of coarse
sand that graded upward to fine sand. These suggested upstream scouring events that mobilized and transported highand coarse-sediment loads episodically, potentially from
the catastrophic drainage of upstream lakes. A number of
DTLBs occur upstream of this site and their drainage dates
are currently unknown, but may correspond to these events.
The basal age of this unit from a sedge fragment yielded a
date of 590 (±30) a BP, considerably younger than we found
at Crea Creek (Fig. 10). A paired sedge and willow macrofossil extracted from above a coarse sand horizon at 20–30 cm
indicated ages of 1430 (±25) a BP and 125 (±25) a BP. Our
interpretation of this core and analyzed ages is that the basal
material was either not reached or had been remobilized and
that a number of very high-flow events in this stream’s recent history had deposited upstream material of varying ages.
These flow events may have partially eroded some of the
late-Holocene record and/or deposited reworked macrofossils, which yielded less certain 14 C ages. The depositional
environments of beaded streams seem discontinuous and difficult to interpret because of unconformities and reworked
plant macrofossils. In the right situation, however, pool sediments may record upstream watershed events such as lake
drainage, as we think is preserved in the Blackfish Creek
core. At this time, the typical lifespan of the beaded streams
we studied remains uncertain, but our best estimate places
the Crea Creek channel’s formation near the Pleistocene–
Holocene transition. The Blackfish Creek core was much
more complicated and provided no apparent clues to the age
of this beaded channel.
Biogeosciences, 12, 29–47, 2015
3.4
3.4.1
Physical processes affecting morphology and
habitat
Winter processes
Because winter is the dominant season in the Arctic and most
beaded streams are ice-covered and likely stop flowing from
October to late May or early June, understanding their state
during this period is of great interest. An important characteristic of beaded stream channels on the ACP is that their often
deep gulches, 0.5–2.0 m, rapidly fill with blowing snow early
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
in the winter, effectively leveling the snow-surface topography with the surrounding tundra. This deep snow insulates
ice on pool surfaces, reducing its rate of thickening, and impacting soil active-layer dynamics as well. Measured snow
depths above beaded streams averaged 122 cm and ranged
from 70 cm on a small pool in Crea Creek to 192 cm above
a pool in Bill’s Creek (both in the lower Fish Creek watershed) (Fig. 11). In contrast, surrounding tundra snowpack
rarely exceeds 40 cm depth by late winter. Not only does
this thick snowpack insulate ice and soil, but it also persists
much longer in the spring and contributes a much larger portion of snow-water per unit area directly to runoff (Arp et al.,
2010). From 12 beads we surveyed from 2010 to 2013, only
one was found to be entirely frozen to the bed by March or
April (Fig. 11). A more detailed and extensive survey of water below ice was conducted in March and April of 2013 using ground-penetrating radar (GPR) and high-resolution synthetic aperture radar (TerraSAR-X) in this area and found
the majority of pools had liquid water below ice (Jones et
al., 2013). The average ice thickness of pools surveyed was
106 cm and ranged from 89 to 129 cm (Fig. 9). For comparison, lake ice thickness in this same region and years ranged
from 118 cm in 2011 to 171 cm in 2013 (Arp et al., 2012a;
Jones et al., 2013). The average depth of water we found
below the ice was 44 cm and ranged from 4 up to 106 cm
(Fig. 11). This water was typically under pressure from ice
expansion and the weight of snow, such that upon drilling
through the ice, water typically floods the frozen pool surface. On at least two occasions live fish (Alaska blackfish,
Dallia pectoralis) were pushed out of the drill hole to the
surface by flowing water during these surveys. Monitored
dissolved oxygen levels in one bead showed a rapid drop
to hypoxic conditions by mid-January and measurements in
March typically showed levels below 5 % of saturation or
< 1 mg L−1 . Alaska blackfish, however, are known to tolerate such conditions (Scott and Crossman, 1973; Crawford,
1974), providing evidence that some beaded stream pools
can function as overwintering habitat for select Arctic fish
species. While we suspect that these stream pools are not
preferred overwintering locations for most fishes, these relatively warm unfrozen sediments may be important habitat
for invertebrate and microbial communities.
Despite the relatively small diameter of pools, thawed sediment underlie most of them and measured depths averaged
120 cm and were up to 170 cm in one pool with sand-gravel
sediment (Fig. 11). Similar talik depths are reported for pools
or broadenings in beaded channels in Russia (Tarbeeva and
Surkov, 2013). This suggests that beaded stream channels
further disrupt the ground thermal regimes of otherwise continuous permafrost landscapes at a scale relative to their size,
whereas large river channels and lakes with floating ice result
in taliks reaching tens of meters or more in depth (Brewer,
1958; Lachenbruch et al., 1962). Since 2009, we have been
monitoring bed temperatures in a set of pools within beaded
stream systems in the lower Fish Creek watershed. Typically
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41
winter bed temperatures rapidly approach the zero-degree
curtain and average winter temperatures (November–April)
consistently average 0 ◦ C (±0.1). Similarly, mean annual bed
temperatures (MABTs) fall within a narrow range averaging 2.9 ◦ C and varying interannually almost entirely according to summer temperatures (Fig. 12a). Such MABTs above
freezing also suggest the presence of a talik (Burn, 2002; Ensom et al., 2012), as we confirmed with field measurements.
The presence of year-round unfrozen sediment and some
liquid water in pools may be an essential factor supporting microbial- and invertebrate-based food webs, which then
feed summer productivity and the use of beaded streams as
important foraging habitat. Additionally, perennially thawed
sediment also likely enhances the survival and productivity
of macrophytes that provide additional habitat and forage.
3.4.2
Summer processes
Much of the variation in the MABT of pools is determined
by whether pools become thermally stratified during the summer. Monitoring of surface temperatures relative to the pool
beds and temperature in the channel runs suggests a wide
range of mixing behaviors and stratification regimes among
pools both between different stream systems and from pool to
pool in a single stream. For example in three beaded streams
monitored from 2009 to 2012, a 1.3 m pool never became
stratified, another 1.4 m pool was stratified by 10 % or more
(i.e., surface temperature / bed temperature > 1.1) for 13 days
per summer on average, and a 2.1 m pool had a stratification ratio of 1.2 and was stratified for over a month on average per year (Fig. 12b). This generally suggests that deeper
pools stratify to a greater degree and for longer periods. To
assess interpool variability, we instrumented an additional
three pools in Crea and Blackfish creeks from June 2013
through August 2013 with surface and bed thermistors. In
Crea Creek with pools depths of 1.6, 1.7, and 2.0 m, the corresponding average stratification ratios (and durations with
ratios > 1.1) were 1.05 (5 days duration), 1.09 (23 days),
and 1.03 (4 days), respectively (Fig. 12b). In Blackfish Creek
with deeper and coalesced pools, instrumented pools had 1.5,
2.2, and 2.6 m depths and the corresponding stratification ratios and durations were 1.04 (5 days), 1.16 (24 days), and
1.10 (19 days). Thus, there is, as expected, some relationship between pool depth and stratification, but this is generally weak and suggests other factors control how water
mixes among different pools. A single densely instrumented
pool in Imnavait Creek was shown to stratify in a complex
and dynamic manner (Merck and Neilson, 2012), similar to
more extensive work completed there originally (Oswood et
al., 1989). The velocity of upstream runs and morphology
of pools at run inflows is certainly one factor. A steeper run
upstream of Bill’s Creek was likely the cause of continuous
mixing during all flows, ambient air temperatures, and wind
regimes, which produced higher MABTs (Fig. 12a) and possibly the deepest talik we measured (Fig. 11).
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 12. Thermal regime characteristics of single pools at three
beaded streams averaged over 4 a (error bars are standard deviations). In 2013, three additional pools within two of these beaded
streams were monitored to assess within-stream variability of thermal characteristics. Thermal regimes were characterized by mean
annual temperatures at pool beds (a) and stratification ratios as the
average ratio between the pool surface and bed during the period
from July to mid-August in each year (b). Pool depths are averaged
during the same period that temperature was summarized in each
plot.
The extent and structure of emergent aquatic macrophytes
in pools likely also plays a role, where some shallow beads
have very dense macrophyte beds (Potamogeton spp., Arctophila fulva, and Hippuris vulgaris are the most common
plants) that likely create a rough and thick boundary layer
enhancing stratification. Adjacent pools of seemingly similar
depth and surface area are often devoid of vegetation, creating greater habitat heterogeneity within beaded stream systems. Variation in water color due to dissolved organic carbon may play some role; however, rarely do beaded streams
in this part of the ACP have highly stained water from organic acids as has been observed in other beaded stream systems at foothill locations (Merck and Neilson, 2012; Oswood
et al., 1989).
Ecologically, the important point in terms of fish habitat is
that within a single beaded stream, varying degrees of mixing and thermal stratification from pool to pool likely create a
range of temperature zones that can be utilized to either avoid
thermal stress or optimize energetics for foraging and other
activities. For example, some salmonids behaviorally thermoregulate by moving to warmer areas after foraging bouts
in cooler water in order to accelerate metabolism and asBiogeosciences, 12, 29–47, 2015
Figure 13. Streamflow hydrographs and temperature regimes for
two beaded streams (Crea (a) and Blackfish (b) creeks) with contrasting channel and watershed morphology. Bed and surface temperatures were monitored in multiple pools within each reach to
document the timing, magnitude, and variation in stratification in
relation to streamflow (streamflow is indicated by QW , temperatures are indicated at pool beds by Tbed and pool surface by Tsur ,
and timing of water tracer injection studies are indicated with red
circles by RWTinj ; all data are presented as mean daily values from
hourly measurements).
similate more quickly (Armstrong et al., 2013). Stratification
within a single bead and heterogeneity in thermal characteristics of nearby beads within a network may provide similar
opportunities to behaviorally optimize growth and foraging
efficiency during summer. This thermal variability may also
play a key role in the distribution of fish prey items, including
the forage fish ninespine stickleback (Pungitius pungitius) as
well as invertebrate and plankton communities (McFarland,
2012).
Similar to the development of stratification in Arctic lakes,
stream pools tend to stratify starting in early July following
snowmelt runoff and associated cold temperatures and turbulent mixing. An episode of intense summer warming leading
to stratification was clearly observed in pools at Crea and
Blackfish creeks starting on 9 July 2013 when the surface
water temperature rose rapidly from 8 to 16 ◦ C over several days while beds warmed more slowly, albeit to differing
degrees (Fig. 13). In Crea Creek, the mean daily temperature
difference between the pool surface and bed was as high as
2.5 ◦ C in one pool and only 0.9 ◦ C in the other (Fig. 13a).
For the same warming event in Blackfish Creek, levels of
stratification were 1.1 ◦ C in one pool and 4.7 ◦ C in the other
(Fig. 13b). Another warming event in late July caused even
higher stratification, up to 5 ◦ C, in pools of both streams.
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 14. Examples of reach-scale water velocity distributions
(reach length/travel time) measured using hydrologic tracer tests
(rhodamine WT pulse additions) shown as cumulative tracer recovered downstream. Results from two beaded streams, Crea Creek
(blue squares) and Blackfish Creek (red triangles) are compared
to an alluvial stream (black circles) in a mountain meadow (Arp
unpublished data, stream described in Arp et al., 2007); all three
streams had similar discharges ranging from 85 to 140 L s−1 during
tracer tests and slopes ranging from 0.1 to 0.2 %, but with otherwise
differing morphologies (experimental data and inverse modeling results shown in Table 3).
In beaded streams on the ACP, we have observed that
peak flows predictably occur only 1–2 days after streams
begin to flow initially, which is first on top of the ice and
often partly beneath the rapidly melting snowpack in stream
gulches. Over 5 a of gauging on five separate beaded streams,
the timing of peak flows ranged from 1 to 10 June with
peak hourly discharges of 1–10 m3 s−1 , which typically exceeds summer flows by 2 orders of magnitude or more. This
fast consistent response is similar to that observed for larger
river systems of the ACP (Arp et al., 2012b; Bowling et al.,
2003), which are fed predominantly by beaded streams and
their source-water lakes. A related characteristic is that water temperatures are very near 0 ◦ C at flow initiation and rise
very rapidly directly following peak discharge, often warming to 10 ◦ C or more over a 2–3 day period (Fig. 13). These
rapid changes in flow and temperature regimes may provide
important cues to fish migrating along larger river courses
fed by beaded streams. Arctic graylings (Thymallus arcticus) are known to seek habitats that warm most rapidly in
the spring to spawn, and the quickly rising temperatures of
beaded streams may contribute to their importance as spawning habitats (Heim, 2014). In fact, we often see individual
fish migrating up beaded channels with water flowing over
bedfast ice just prior to peak flows, when their dark bodies
can be easily observed crossing the white ice surface. Tracking studies of Arctic grayling tagged in Crea Creek, show a
rapid pulse of upstream migration into the system during and
after peak flow (Heim, 2014). This early upstream migration
may represent an adaption to maximize time spent in producwww.biogeosciences.net/12/29/2015/
43
tive spawning habitats at the earliest possible time in order to
provide a longer period of growth for offspring.
More broadly, the period of peak flow across this hydrologic landscape represents a period of high connectivity
among aquatic habitats, where fish can disperse from relatively limited deepwater overwintering habitats and move
into shallow, seasonally flowing habitats like beaded streams.
Again, in late August through September, changes in flow
and temperature may become important environmental cues
that fish use to time migratory movements out of beaded
streams (Heim, 2014). Migration out of Crea Creek in the fall
was strongly correlated to decreases in stream temperature,
as the channel connection to the Ublutuoch River became
restricted due to ice formation. Low flows and colder temperatures increase the risks of utilizing Crea Creek (Arctic
grayling were not found to overwinter within the drainage),
yet persistence of fish within the drainage through September may be advantageous in terms of growth and acquisition
of energy reserves prior to the onset of winter (Heim, 2014).
With respect to the basic physics of flow through stream
systems characterized by multiple evenly spaced pools (storage zones), the attenuation of flows seems intuitive. This has
implications for streamflow dynamics, movement and transformations of solutes (carbon, nutrients, and contaminants),
the transport of particles including mineral and organic sediment, plankton (both semimobile and drift), and the movement of fish. Because most beaded streams are set within a
permafrost framework without interactions with groundwater systems, the development of hyporheic flow through bed
material or banks is unlikely. Storage processes have been
investigated in Imnavait Creek and adjacent beaded streams
around Toolik Lake in Alaska where the glaciated setting
and corresponding porous substrates, and known spring systems, may allow hyporheic storage to play a significant role
in beaded stream hydrology (Merck et al., 2012; Zarnetske
et al., 2007). Still, we suggest that the characteristic large
size and frequency of pools of beaded streams strongly dominates transient storage, even when groundwater systems are
present and allowing for hyporheic exchange, which is probably rare in continuous permafrost zones of the ACP where
surface-water interactions with groundwater are absent.
The distribution of water velocity at the reach scale in a
beaded stream with large, deep and coalesced pools (Blackfish Creek) compared to a stream with shallower elliptical
pools (Crea Creek) using tracer tests highlights how such
morphology functions in water storage and residence time
(Fig. 14). For example, the much more rapid velocities observed in an alluvial channel with otherwise similar discharge and slope underscores this impact on dense, evenly
spaced pools on the hydrologic functioning of beaded channels. A similar range of reach-scale velocities are reported
when comparing beaded channels to other channel types in
Arctic drainage networks (Tarbeev and Surkov, 2013, Zarnetske et al., 2007).
Biogeosciences, 12, 29–47, 2015
44
C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Table 3. Results from reach-scale tracer injections for Crea (325 m, shallow elliptical beads) and Blackfish (232 m, deep coalesced beads)
creeks during the summer of 2013 (RWT is rhodamine WT, Q is stream discharge, A is the advective cross-sectional area, U is advective
zone velocity, D is the dispersion coefficient, AS is the storage zone cross-sectional area, AS /A is the relative storage zone area, α is the
storage zone exchange coefficient, AR and UR are the cross-sectional area and velocity, respectively, of a single channel run). Comparisons
of these results are made to two other RWT tracer studies of similar sized streams with beaded and other channel morphologies.
Site
Experiment data
Date
Solute added
(RWT, g)
Total channel hydraulics
Q
A
U
D
(m3 s−1 ) (m2 ) (m s−1 ) (m2 s−1 )
Channel storage zone
AS
AS /A
α
(m2 )
(s−1 )
Channel run (single)
AR
UR
(m2 )
(m s−1 )
beaded streams in Fish Creek watershed in 2013 (this study)
Crea
Crea
Crea
Blackfish
Blackfish
Blackfish
14-Jun
5-Jul
25-Aug
13-Jun
6-Jul
24-Aug
70.4
49.9
19.9
92.4
41.4
19.1
Lake inlet
Lake outlet
Beaded
Beaded
17-Jun
–
0.26
–
0.16
1.48
–
–
2.0E-4
–
–
18-Jun
–
0.09
–
0.07
1.71
–
–
5.0E-4
–
–
25-Jun
–
0.05
–
0.02
1.75
–
–
3.0E-4
–
–
21-Jun
–
0.44
–
0.09
1.94
–
–
6.0E-4
–
–
multiple stream types in a mountain meadow in 2004 (Arp unpublished data, streams described in Arp et al., 2007)
Alluvial
Lake outlet
11-Aug
10-Aug
–
–
1.17
5.29
0.22
3.33
5.32
1.01
1.9E-03
0.13
1.89
0.04
0.88
2.71
1.43
5.9E-04
0.03
1.95
0.01
0.38
2.55
1.31
1.2E-03
1.73
9.81
0.18
1.90
9.08
0.93
3.2E-03
0.09
7.00
0.01
0.45
6.60
0.94
1.5E-03
0.03
–
–
–
–
–
–
multiple stream types near Toolik Lake in 2004 (Zarnetske et al., 2007)
0.14
0.17
–
–
0.22
0.06
Residence times of water in these two beaded channels
increase predictably with decreasing flows and relatively
higher storage areas (Table 3). At the start of peak-flow recession, over 10 % of the water in both channels was still moving
at velocities lower than 0.1 m s−1 . During summer flows, the
fastest reach-scale velocities did not exceed 0.2 m s−1 in Crea
Creak and 0.05 m s−1 in Blackfish Creek. Even though individual run velocities often exceed 0.5 m s−1 or greater, the
water in the channel exchanges with storage zones (pools)
sufficiently to slow the total movement of water by up to
an order of magnitude or much more. Such slow transport
rates of water in beaded stream systems may have important
implications for maintaining in-stream flow during dry summers when evapotranspiration far exceeds rainfall on daily
to weekly timescales. The major source of water to these
channels are upstream lakes (Arp et al., 2012b; Bowling et
al., 2003), and the evenly spaced storage-rich nature of these
streams may function to maintain more constant flows and
reduce evaporative losses during summer drought periods.
The summer of 2013 when these experiments were conducted was very wet and rainy compared to previous years
when we have monitored discharge in these streams. Still,
in 5 a of monitoring, starting in the summer of 2008, we
have not yet observed interruptions in flow during summer
drought periods in the five gauged streams. At least some
alluvial streams in the Arctic foothills of Alaska have experienced prolonged periods of no flow over certain reaches
during drought conditions when only minimal flow through
Biogeosciences, 12, 29–47, 2015
–
–
–
–
0.69
0.23
1.6E-4
6.7E-4
2.48
0.43
0.10
2.90
0.52
0.36
–
–
0.54
0.39
0.33
0.70
0.33
0.15
–
–
interstitial gravels disrupt migration of the Arctic grayling
(Betts and Kane, 2011). In some instances, individual Arctic
graylings have been observed traveling over 160 km within
a year visiting different key habitats within a “migratory
circuit” (West et al., 1992). Thus, connectivity among spatially separated habitats is critical to this life history strategy,
and beaded streams may function in maintaining hydrologic
connectivity and fish passage between alluvial rivers, tundra
lakes and ponds. Extreme drought conditions occurred on the
ACP and foothills during the summer of 2007 and the hydrologic response has been well documented in rivers (Betts and
Kane, 2011; Arp et al., 2012b), thermokarst lakes (Jones et
al., 2009a), and upland tundra (Jones et al., 2009b) in this
region. Whether beaded streams in this area maintained hydrologic connectivity between river and lake systems through
this dry summer was undocumented and warrants reconstruction through hindcast modeling.
The other key function that the hydraulics of beaded
streams provides is productive foraging habitat for Arctic
fishes. This stems from the observation that larger foraging fishes (e.g., Arctic grayling) spend much of their time
holding in channel runs downstream of pools, where they
efficiently ambush drifting zooplankton, invertebrates, and
ninespine stickleback (McFarland, 2012). The rapid shift
in velocities from pools to runs may function as a key
delivery system of forage that either resides primarily in
beaded stream pools (i.e., ninespine stickleback and aquatic
macroinvertebrates) or comes downstream as drift from lakes
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C. D. Arp et al.: Distribution and biophysical processes of beaded streams
Figure 15. Conceptual diagram showing morphology, physical
regimes, habitats, and organisms of a hypothetical pool-run system
in the summer (a) and winter (b) based on observations and monitoring studies in multiple beaded stream systems during these time
periods over many years.
(i.e., zooplankton) or laterally from riparian vegetation (i.e.,
terrestrial invertebrates). Such a setting may in part be the
same reason why lake inlets and outlets are such productive
ecosystems (Jones, 2010). The difference here is that along
the course of beaded streams, this lake outlet delivery system
is replicated multiple times over a short distance (i.e., 5 times
per 100 m on average; Fig. 6). Approximately half of the Fish
Creek drainage network is composed of beaded streams, the
equivalent of 1200 km of stream length (Arp et al., 2012b).
If we assume a pool density of five per 100 m, this gives us
an estimated 60 000 pools (beads) throughout this watershed.
Recently, the development of a Fish Creek watershed classification of lakes > 1 ha shows 4362 lakes, of which 45 %
have perennial stream outlets and another 30 % have at least
ephemeral outlets (B. M. Jones, unpublished data). In terms
of potential fish habitat for summer foraging, this comparison
suggests that pools in beaded streams increase the number of
potential fish habitat zones for ambush foraging by 18-fold
across the landscape over lake inlets and outlets alone.
4
Conclusions
This body of research on beaded streams in continuous permafrost landscapes documents a wide and varied distribution across the circum-Arctic in relation to ground-ice content in the upper permafrost, topography, and elevation. On
the inner coastal plain of northern Alaska, our surveys indicate that beaded streams compose the majority of drainage
networks and most channels initiate from and are fed by
lakes. At least in northern Alaska, lakes supply water for
new development in the form of ice roads and other industrial and municipal uses. Knowing how such practices afwww.biogeosciences.net/12/29/2015/
45
fect downstream ecosystems warrants investigation. Channels with beaded morphology are maintained downstream,
eventually forming alluvial channels in relation to varied water and sediment supply. This suggests that new land disturbances, such as road construction or thermokarst processes
that can alter these watershed fluxes, will factor into future
drainage network changes. It also appears that beaded stream
channels are relatively stable over time and potentially very
old, such that any observations of rapid channel change may
be indicative of more extreme forcing agents, either anthropogenic or climate driven. Given these concerns and the high
density of beaded stream systems in many Arctic landscapes,
expanded research into the role of these ecosystems in permafrost, hydrological, and biological processes will be essential.
The coupled biophysical processes of beaded stream systems that provide key ecosystem functionality are described
conceptually in Fig. 15. We found high spatial and temporal thermal variability among pools, which likely play an
important role in permafrost thaw and cold-water habitats
(Fig. 15a). Beaded morphology appears to also play an important role in summer feeding habitats and hydrologic connectivity for migrating fish, the quality and availability of
which is critical during short Arctic summers. During long
Arctic winters, beaded stream gulches fill with deep snow
that effectively insulates ice and permafrost and plays a role
in creating taliks and providing overwintering habitats for
certain fish and invertebrate communities (Fig. 15b). This
conceptual understanding of beaded stream systems helps
summarize seasonal and reach-scale ecosystem functions of
interest to physical and biological scientists including managers concerned with changing human uses of Arctic lands
and waters.
Acknowledgements. This research was supported primarily by the
Bureau of Land Management’s Arctic Field Office and the Arctic
Landscape Conservation Cooperative. Additional funding was
provided by Alaska EPSCoR Northern Test Case (OIA-1208927),
the Circum-Arctic Lake Observation Network (ARC-1107481),
and the National Fish and Wildlife Foundation. We thank F. Urban,
R. Kemnitz, C. Couvillion, M. Lilly, J. Adams, J. Derry, H. Toniolo,
J. Webster, V. Alexeev, and J. McFarland along with numerous
helicopter pilots and ConocoPhilips-Alaska, Inc. (Alpine Facility)
for assistance with fieldwork and logistics. This research benefited
early on from enlightened conversations with Sveta Stuefer and
Matthew Sturm. Two anonymous reviewers provided thoughtful and welcomed comments that improved this manuscript.
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
Edited by: J. Vonk
Biogeosciences, 12, 29–47, 2015
46
C. D. Arp et al.: Distribution and biophysical processes of beaded streams
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