Beaver dams affect hydrologic processes, channel complexity, and stream
temperature in part by inundating riparian areas, influencing
groundwater–surface water interactions, and changing fluvial processes
within stream systems. We explored the impacts of beaver dams on hydrologic
and temperature regimes at different spatial and temporal scales within a
mountain stream in northern Utah over a 3-year period spanning pre- and
post-beaver colonization. Using continuous stream discharge, stream
temperature, synoptic tracer experiments, and groundwater elevation
measurements, we documented pre-beaver conditions in the first year of the
study. In the second year, we captured the initial effects of three beaver
dams, while the third year included the effects of ten dams. After beaver
colonization, reach-scale (
Beaver dams create ponds that change surface water elevations, alter channel morphology, and decrease flow velocities (Gurnell, 1998; Meentemeyer and Butler, 1999; Pollock et al., 2007; Rosell et al., 2005). These ponds and the overflow side channels are forced by high dam crest elevations and a general increase in water storage, water residence time, and depositional areas for sediments. The increased storage attenuates hydrographs (Gurnell, 1998) and can increase base flow (Nyssen et al., 2011). Specifically in the beaver ponds, water infiltration through the streambed and adjacent banks influences local groundwater elevations (Hill and Duval, 2009). Within the stream channel, beaver dams break up the average hydraulic gradient into a series of disrupted head drops and flat ponded sections. This change in average hydraulic gradient increases the potential for hyporheic exchange (Lautz and Siegel, 2006). Such changes in channel morphology and hydrology alter stream temperature regimes. Warming due to solar radiation can be a key factor due to increased water surface area (Cook, 1940) and changes in morphology that influence solar radiation fate within the water column and penetration to the bed sediments (Snow, 2014; Neilson et al., 2009; Merck et al., 2012). Foraging and extensive inundation can lead to loss of riparian vegetation that decreases riparian canopy and the associated shading influences (Beschta et al., 1987). Changes in groundwater–surface water interactions can also impact the overall temperature regime (e.g., upwelling zones decrease temperatures below beaver dams; Fanelli and Lautz, 2008; White, 1990). Regardless of this implied connection between hydrologic and stream temperature changes due to beaver dam construction, most studies have investigated these changes separately. Furthermore, the temporal and spatial scales considered within individual studies vary widely, leading to inconsistent conclusions regarding beaver dam impacts on stream systems (Kemp et al., 2012).
When considering hydrologic influences at the beaver dam scale (which includes the beaver dam structure, the upstream ponded area, and the section below the dam), Briggs et al. (2012) found a connection between streambed morphologies formed upstream of a beaver pond and the hyporheic flow patterns. Similarly, Lautz and Siegel (2006) showed that beaver dams promoted higher infiltration of surface water into the subsurface. Janzen and Westbrook (2011) found enhanced vertical recharge between the stream and underlying aquifer upstream of dams and longer hyporheic flow paths than those measured in other studies. Nyssen et al. (2011) studied impacts of beaver dams at a larger reach scale and throughout a series of beaver dams; similar to other literature (Gurnell, 1998; Burns and McDonnell, 1998), they found that a series of beaver dams retained water during high flows and increased low flows through drier periods. The authors also assessed that the recurrence interval for major floods increased over 20 years and peak flows were decreased and delayed by approximately 1 day. In contrast, some argue that while beaver dams affect downstream delivery of water, they provide minimal retention during extreme runoff events (Burns and McDonnell, 1998).
The documented impacts of beaver dams on temperature are more variable. Some
studies found that beaver dams and beaver ponds cause overall increases in
downstream temperatures (Andersen et al., 2011; Margolis et al., 2001;
Salyer, 1935; McRae and Edwards, 1994; Shetter and Whalls, 1955) with
reported values as high as 9
Variability in hydrologic and thermal responses in streams with beaver dams and the subsequent inconsistent conclusions found in the literature highlight the need for more data driven studies across multiple spatial and temporal scales. In an effort to link hydrologic and temperature responses due to beaver dam development, we present data from different spatial (reach, sub-reach, and beaver dam) and temporal scales (instantaneous to continuous 3-year time series) that span a period prior to and during the establishment of 10 beaver dams. We illustrate how the development of beaver dams shifts instream hydrologic and thermal responses.
Aerial image from 2006 (pre-beaver period) and beaver dams constructed between 2009 and 2010. The main beaver dams are numbered 1–10 from upstream to downstream and the time of dam construction is noted in the table. The study reach was further divided into six sub-reaches. The spatial scales investigated are illustrated below the map. The most downstream beaver dam and beaver pond are located in the old channel but overlap in the beaver dam scale schematic in this figure. The 2006 channel is outlined in black, while the flowing and ponded water area from 2010 are represented by different shades of blue.
Curtis Creek, a tributary of the Blacksmith Fork River in northern Utah, drains a portion of the Bear River range. Curtis Creek is a first-order perennial mountain stream with intermittent tributaries. The mountainous watershed includes a combination of hard sedimentary rock; Paleozoic and Precambrian limestone bedrock that is strongly indurated. The valley broadens in the lower portion of Curtis Creek and is primarily dominated by remnant low-angle alluvial fans. The valley bottom is comprised of a mix of longitudinally stepped floodplain surfaces and channels that are both partly confined by coarse-grained alluvial fan deposits with gravel, cobble, boulders and some soil development.
Data were gathered in a 750 m long study site on the lower portion of Curtis Creek that is located about 25 km east of Hyrum, Utah, at Hardware Ranch (an elk refuge operated by the UDWR). In 2001, the UDWR conducted a stream relocation project within the study reach and some segments of the channel were moved and reconstructed, leaving portions of the original channel abandoned. The study reach has an average streambed slope of 0.017 with steeper riffles, riffle-step sequences, milder beaver pond sections and small meanders that support a streambed of coarse gravel to large cobble with some man-made boulder vortex weirs placed within the new channel. The banks of the realigned channel were stabilized with boulders, root wads, logs, and erosion control blankets.
The riparian area surrounding the channel prior to and following relocation
was heavily grazed by elk and did not support woody riparian vegetation.
Around 2005, grazing pressure was lessened and the area was fenced (though
some grazing was still allowed). This facilitated some modest recovery of
the riparian woody vegetation which was enough to attract beaver. In early
summer of 2009, beaver colonization began with beaver dam 7 being
constructed in the middle of the study reach (Fig. 1). Beaver dams 4 and 5
were also completed during the summer of 2009. New beaver dams (3 and 8)
were established early summer 2010 and by the late summer–early fall, dams
2, 6, 9, and 10 were completed. By the end of fall 2010, beaver dam 1 was
built at the upstream end of the study reach resulting in a total of 10
beaver dams with an average height of 1 m (measured at the downstream face
of a dam as the difference between the channel bottom and the top of the dam
crest). In addition, two small (less than 0.5 m in height) beaver dams were
constructed in the old channel (Fig. 1; dams without numbers). Beaver built
seven of their dams using the artificial restoration structures as
foundations. By the end of fall 2010, the channel consisted of sections with
flowing water (main channel and side channels), ponded water (beaver ponds),
and beaver dam structures (Fig. 1). The resulting dam density by 2010 was
13.3
The field site was originally instrumented with pressure transducers,
temperature sensors, and groundwater observation wells to investigate
groundwater–surface water interactions in the absence of beaver. After 1
year of data collection, beaver colonization occurred within the study reach,
changing the objectives of the study. In short, it produced the perfect
accidental experiment and a unique opportunity to quantify fundamental
hydrologic and thermal impacts of beaver dam construction on stream systems.
In an effort to specifically investigate these impacts, three primary data
types were collected over a 3-year period spanning pre- and post-beaver
colonization (Table 1; Fig. 1). Flow information was collected at the reach
(
Discharge, temperature, and ground water level observations made at different spatial and temporal scales throughout the study reach.
The study reach boundaries were set following a previous study (Schmadel et
al., 2010) and locations along the reach were denoted by distance downstream
from an arbitrary datum set upstream of the study reach (Fig. 1). Water level
and temperature were measured using KWK
Technologies®
SPXD™ 610 (0–5 psig) (Spokane, Washington)
pressure transducers (PT) with vented cables and Campbell
Scientific® CR-206 data loggers (Logan, Utah)
at the upstream, inflow (PT515; Fig. 1) and downstream, outflow study reach
limit (PT1252; Fig. 1). Both pressure transducers were installed in the
flowing water close to the bank with an average streambed slope of 0.017 and
0.024 for inflow (PT515) and outflow (PT1252), respectively. Water level and
temperature were measured at 30 s intervals and 5 min averages were
recorded. Discharges were measured at each PT under the full range of flow
conditions using the velocity-area method to establish rating curves. The
flow velocity was recorded with a Marsh McBirney
Inc.®
Flo-Mate™ (Model 2000, Frederick, Maryland).
The lowest flow measured was 157
The study reach was further divided into six sub-reaches, ranging from 56 to
168 m and numbered sequentially downstream (Fig. 1). The six sub-reaches
spanned individual dams (e.g., sub-reach 4), multiple dams (e.g.,
sub-reaches 2 and 5), and a non-impounded sub-reach that received surface
return flows via small side channels or overland flow from an upstream beaver
pond (sub-reach 3). The boundaries for the sub-reaches were chosen to ensure
completely mixed conditions necessary for dilution gaging (Schmadel et
al., 2010). Dilution gaging was conducted at the sub-reach scale on 16 July
2008 (pre-beaver) and 19 July 2010 (post-beaver) to provide a longitudinal
understanding of flow variability. As described within Schmadel et al. (2010,
2014), chloride (from NaCl) was used as a conservative tracer (Zellweger,
1994) and rhodamine WT was used as a visual indicator for a qualitative
assessment of mixing. Tracer injection masses ranged from 600 to 3300 g as
NaCl and were varied to achieve large enough responses in electrical
conductivity above background for dilution gaging and mass recovery purposes.
Tracer responses were measured following an instantaneous tracer injection
starting at the downstream end of the study reach and then moving upstream to
individual sub-reach boundaries. Each response was measured with specific
conductance (SC) (electrical conductivity normalized to 25
To capture changes in groundwater levels throughout the reach, groundwater observation wells were installed in June 2008 (Fig. 1). These wells were constructed of half-inch polyvinyl chloride (PVC), 2 m in length with 40 cm of perforation covered with 2 mm flexible nylon screen to exclude soil. Elevations and horizontal coordinates were established for individual wells using differential rtkGPS (Trimble® R8, Global Navigation Satellite System, Dayton, Ohio). Groundwater levels were determined by measuring the distance from the top of each well to the groundwater surface level in each well using a Solinst® electronic well sounder (Model 101 Mini, Georgetown, Ontario, Canada). The groundwater levels were measured 4 times in 2008 (June, July (twice), August), 5 times in 2009 (June, July, August (twice), and November), and 4 times in 2011 (April, June, July, and November).
Distance for temperature sensors located upstream and downstream of individual beaver dams (BD) during 2 September–15 October 2010 (Fig. 1).
At the finer beaver dam scale, temperature measurements were collected upstream of ponded water of beaver dams and downstream of individual beaver dams at 10 min intervals using Onset® HOBO® Temp Pro V2 (Bourne, Massachusetts) deployed from 2 September to 15 October 2010 (Fig. 1; Tables 1, 2). The temperature sensors were placed in the thalweg of the flowing channel entering the pond to ensure well-mixed conditions. The sensors downstream from the beaver dams were placed past the scour pool, but in the completely mixed portion of the channel. The temperature sensors were attached to metal stakes, placed in the channel, approximately halfway through the water column. Individual sensors were wrapped in aluminum foil to reduce solar radiation influence in slower moving waters.
The air temperature, relative humidity, solar radiation, wind speed and precipitation data were collected with the meteorological station located within the study reach.
Aerial imagery was used to delineate and compare pre- and post-beaver colonization flowing and ponded water area. Pre-beaver colonization conditions (2006) were captured with high-resolution aerial imagery available through the Utah Automated Geographic Reference Center (AGRC). Post-colonization, NIR (near infrared) and RGB (red-green-blue) aerial imagery were collected using Aggie Air UAVs (Unmanned Aerial Vehicle) in 2010. Aggie Air flights that additionally included thermal aerial images were completed in 2011–2013.
At the reach scale, the 5 min continuous stage and temperature data
recorded at the study reach boundaries were averaged to daily values to
illustrate changes over the 3-year study period. Data from the winter
months were excluded from the analysis because they were influenced by ice
buildup around the pressure transducers. Rating curves were developed from
the measured discharges and continuous stage in the form (Cey et al., 1998;
Rantz, 1982):
At the finer, sub-reach scale, stream discharge was calculated at each
sub-reach limit from dilution gaging using (Kilpatrick and Cobb,
1985)
The temperature impacts at the beaver dam scale were quantified from the data
collected upstream of ponded waters and downstream of individual beaver dams
(3, 4, 5, 7, and 8) from fall 2010 (Fig. 1 and Table 2). In the case of
beaver dams 7 and 8, the ponded water from beaver dam 8 extended to beaver
dam 7. Therefore, we used data upstream of dam 7 and downstream of dam 8. A
24 h moving average was calculated from the data to detect temporal trends
other than diurnal patterns. The net temperature change,
At the reach scale, the average daily discharge (Fig. 2) illustrates the
seasonal variations and changes in flow conditions at the inflow (PT515) and
outflow PT1252 for 2008 through 2010. The 2008 and 2009 flows were fairly
comparable with peak flows at PT1252 of 1698 and 1549
Annual change in flow (
Daily average discharge estimated from continuous pressure
transducer records spanning 2008–2010
Across shorter temporal scales, variability within each season of each year
was also apparent. Even though data are only available for a short portion of
the spring period in 2008, the reach was gaining. In July 2008, the
%
Average daily temperature (absolute) representing reach-scale
responses at inflow (PT515, black-dashed line) and outflow (PT1252, red-solid
line) during
At the reach scale, stream temperatures consistently increased during the
summer with peaks occurring at the end of July and beginning of August with
some periods of cooling within the reach in the fall for all 3
years (Fig. 4). Net and percent changes in temperature (
The one-way ANOVA for air temperature comparison showed no statistical
difference between individual years (
Reach-scale data from a smaller temporal scale (a 5-day period in July)
illustrates the links between discharge and temperature patterns associated
with beaver dam construction (Fig. 6). Comparison of
With an increase in the number of beaver dams for each consecutive year, the groundwater elevation increased in sub-reaches as shown by the changes in the annual distribution and median values (Figs. 7; S2). The response was greatest for sub-reach 2, where median groundwater levels increased approximately 0.03 m during the first year (2008–2009) and by another 0.34 m from 2009 to 2011. For sub-reaches 3 and 5, median groundwater levels increased by 0.02 and 0.12 m from 2008 to 2009, respectively. From 2009 to 2011, these levels increased further by 0.10 m in sub-reach 3 and by 0.15 m in sub-reach 5. Based on the positive head gradient between groundwater and surface water, sub-reaches 2 and 3 are primarily gaining. However, sub-reach 5 is generally neutral in 2008 and is more commonly losing surface water in 2009 and 2010 (Figs. 7; S2). The head gradients from the cross section of wells in sub-reach 5 show an increase in groundwater elevation over time and generally depict a positive gradient on one side of the channel and negative gradient on the other (Fig. S2).
Change in discharge (
Groundwater elevations grouped by individual sub-reaches and shown with channel water surface elevations. The groundwater elevations were measured 4 times in 2008, 5 times in 2009, and 4 times in 2011. The water surface elevation in the channel represents the average yearly value for each sub-reach. There is a gradual increase in groundwater elevation and channel water surface elevation in all sub-reaches over the years.
Groundwater–surface water exchanges throughout the study reach prior to beaver dam
influences were documented in Schmadel et al. (2014). Discharge estimated at
various locations longitudinally illustrates the variability in flows prior
to beaver dam influences (Fig. 8a) and the sub-reach-scale %
Mean residence times estimated from the 2008 and 2010 tracer studies show an increase for all sub-reaches containing beaver dams (Table 4). The biggest change was observed in sub-reach 2 where beaver dam 4, with the largest pond area, was located (Fig. 1). The second greatest increase occurred in sub-reach 5 where a series of dams and ponds covered approximately 50 % of the sub-reach length. The increase in sub-reach scale residence times translates into an overall reach scale increase of 62 min or 230 %.
Sub-reach-scale mean residence times for 2008 and 2010.
The spatial and temporal temperature differences observed between individual
beaver dams from a 2-day period show that each dam influences the system
differently throughout each day (Fig. 9). A comparison of absolute
temperatures above and below individual beaver dams, where a positive change
represents net warming and negative change represents net cooling below the
beaver dam, illustrates a general downstream warming trend which cumulatively
propagated downstream below beaver dam 8 (Fig. S3). Although the temperature
increase for each dam was generally within the accuracy of the temperature
sensor (
Based on the data shown within Fig. 10, daily ranges (daily maximum minus
daily minimum values) of temperature differences below and above each beaver
dam (
While many studies exist regarding the influence of beaver dams on the local hydrologic and temperature regimes, the majority of these studies lack sufficient field measurements across appropriate spatial (beaver dam to reach scale) and temporal scales (instantaneous to continuous over a period of years) to draw meaningful conclusions (Kemp et al., 2012; Gibson and Olden, 2014). Furthermore, the results are often inappropriately generalized beyond the scales of the observations. Our observations provide an opportunity to quantify the influences of beaver dams on stream flow and temperatures, while demonstrating how beaver dams impact stream hydrologic and temperature regimes at different spatial and temporal scales.
The reach-scale results of our study suggest an overall increase in
Sub-reach stream discharge (
When considering the smaller spatial scales (sub-reach, beaver dam) there is
great variability in terms of losses and gains that are not fully understood
from the reach-scale observations in the study reach with beaver dams
(Figs. 7, 8, Table 4). This variability is due to many different mechanisms
occurring in and around beaver dams, including groundwater–surface water
exchanges (Lautz and Siegel, 2006; Janzen and Westbrook, 2011). However, the
sub-reach-scale variability in this study (Fig. 8) was primarily due to high
crest dams forcing year round overbank flow. Much of the overbank flow was
either returned to the main channel through side channels or was diverted to
the off-channel beaver ponds. These changes in flow paths influenced the mass
recovery in our tracer study in 2010 and the highest mass loss occurred in
sub-reaches with big beaver dams and multiple side channels. Furthermore, the
window of detection inherent to tracer experiments (i.e., the elapsed time
over which the tracer is observed) varies as a function of stream
characteristics, such as transient storage volume, and stream velocity and
discharge (Harvey and Wagner, 2000). In turn, any tracer mass not recovered
within the window of detection will be considered a permanent loss even if
some mass eventually returns to the stream (e.g., Ward et al., 2013). Because
the changes to the study reach between years influenced the window of
detection and, therefore, the reported mass recoveries, our conclusions are
primarily based on the net changes to flow (%
The dynamic activity of beaver, through construction and maintenance of dams, and natural seasonal changes in flow led to a diverse range of hydrologic responses resulting in the spatial and temporal variability of gains and losses through the study reach. The dilution gaging results show that at the two points in time we sampled, sub-reach 2 transitioned from gaining to losing (Fig. 8). However, if groundwater and channel surface water elevation data are aggregated over a year, the same reach was shown to be dominantly gaining over the study period (Fig. 7). These differing results from dilution gaging and groundwater levels highlight the importance of temporal scales and repeated measurements considered in this present work. They also indicate that without this consideration, the differences between measurement techniques can lead to contradicting conclusions as discussed within Schmadel et al. (2014). It is also important to note that the positive head gradients on river left (in a downstream direction) illustrate why sub-reach 5 is gaining water (Figs. 7; S2). However, it is also likely losing water on river right. Sub-reach 6 is gaining water due to both the main and side channels meeting again (Figs. 1, 8).
Spatial variability in stream temperature throughout individual
beaver dams (BD). Temperature differences (
Our temperature results demonstrate the considerable spatial and temporal variability in stream temperature caused by beaver dams. We captured the warming effect at the reach scale over a period of 3 years (Figs. 4, 5). However, the data at this scale do not portray the thermal heterogeneity illustrated by the beaver dam scale temperatures (Figs. 9, 10). Similarly, the temporal scale is of importance when determining impacts of beaver dams. For example, the 5 min temperature data captured temperature fluctuations during the day that may play an important role in fish habitat management and restoration (Fig. 6c–d). This daily variability would not be captured if only daily averages or instantaneous measurements were recorded. The lag times in peak temperatures from 2008 to 2010 (more apparent at shorter temporal scales (e.g., Fig. S4)) are likely due to different flow conditions, air temperatures, solar radiation, precipitation, and channel morphology.
To understand the significance of simultaneously considering the spatial and
temporal scale of measurements, Figs. 9–10 illustrate the temperature
variability for five beaver dams while providing a comparison between the
dams. Individual beaver dams introduce more variability than that observed at
the reach scale with warming and/or cooling effects during different times of
the day. These individual responses are likely due to the diverse beaver dam
morphology, size of the beaver dam, and size of the beaver pond (Fuller and
Peckarsky, 2011; McGraw, 1987). However, considering a longer temporal scale,
the temperature variability associated with a 24 h moving average falls
within a measurement error (
With the transition from a losing to gaining reach, one might expect a decrease in temperature during the summer due to the addition of colder groundwater. However, we observed increased warming over the study reach. Based on this expectation that a gaining reach should be cooling, it is important to discuss the different heat transfer mechanisms influencing instream temperature responses. It is well established that surface heat fluxes (shortwave radiation, incoming and outgoing long-wave radiation, conduction/convection, and evaporation/condensation) and subsurface processes (e.g., bed conduction, groundwater/hyporheic exchanges) are often the primary factors dictating stream temperature responses (e.g., Cardenas et al., 2014; Evans et al., 1998; Moore et al., 2005; Neilson et al., 2010a, b; Sinokrot and Stefan, 1993; Webb and Zhang, 1997; Westhoff et al., 2007; Younus et al., 2000). When considering the transition between pre- and post-beaver colonization, the doubling of the channel surface area is critical because surface heat fluxes are scaled with the area (Neilson et al., 2010a). The influence of these fluxes on temperature is also dependent on the difference in the volume of water in the channel and the residence time within the study reach. Based on the observed temperature increases, the doubling of the surface area (Fig. 1; Table 3) and the tripling of the residence time (Table 4) negate the buffering effects of an almost quadrupled main channel water volume (Table 3) and the cooling effects associated with groundwater inflows. As found within other prior studies, the general downstream warming is due primarily to influences of solar radiation (Cook, 1940; Evans et al., 1998; Johnson, 2004; Webb and Zhang, 1997).
Regardless of the larger scale downstream trends, it is critical to consider
smaller scale thermal heterogeneity. To illustrate the thermal heterogeneity
and complexity of flow paths resulting from beaver colonization, a thermal
image of surface stream temperature in May 2012 shows that temperatures range
from 11 to 18
This study emphasizes the need to understand the variability in flow and temperatures at different spatial and temporal scales. Furthermore, these data begin to provide an explanation as to why the current literature provides inconsistent information regarding the influences of beaver colonization. Although it is difficult to make any generalizations about the hydrologic and thermal impacts of beaver dams (e.g., beaver dams increase temperature), we measured an increased variability in flow and temperature that have been qualitatively discussed in previous studies. Our quantification of the variability across different spatial and temporal scales provides a context for better interpreting the inconsistent information found in the literature. In a given locality or under specific circumstances, we contend that the patterns of increasing variability in flows and temperatures should create and maintain more heterogeneous habitat that has a greater probability of providing multiple niches and supporting greater biodiversity. We believe that this observed hydrologic and thermal variability is an important and more generalizable attribute of beaver dams. Variability in temperature, flow properties, and the associated increase in microhabitat complexity are often restoration goals. However, if beaver are being considered as a restoration tool (e.g., Utah Beaver Management Plan), the importance of further understanding and predicting their impacts on stream systems at different spatial and temporal scales is a necessity. Based on these findings, future efforts in understanding the impacts of beaver dams on hydrologic and temperature regimes should begin by identifying the spatial and temporal scales of data required to address specific questions and/or restoration goals. Ultimately, more quantitative field and modeling studies are needed to fully understand impacts of beaver on stream ecosystems for the potential use of beaver as a restoration tool.
This study quantifies the impacts of beaver on stream hydrologic and temperature regimes, and highlights the importance of understanding the spatial and temporal scales of those impacts. Based on the flow and temperature data collected over the period of pre- and post-beaver colonization, we found a general increase in stream discharge and stream temperatures at the reach scale. The reach transitioned from slightly losing in 2008 (pre-beaver colonization period) to gaining in 2010 (post-beaver, second year into beaver colonization). Similarly, we observed a downstream warming effect over the 3-year study period. We found that the reach-scale hydrologic and temperature changes do not reflect the variability captured at smaller sub-reach and beaver dam scales. For example, temperature measurements at finer temporal scales (5–10 min records throughout each day) revealed significant within-day variability at smaller spatial scales that was not captured at the reach scale. Our most important and likely transferable findings are with regards to the increase in hydrologic and thermal variability that beaver dams produce. We captured natural variability of hydrologic and thermal processes at the sub-reach scale prior to beaver dam influences and show how this variability increased after beaver colonization. While some sub-reaches showed gaining trends from 2008 to 2010, some began losing due to flow being rerouted by dam construction. In addition, daily stream temperature variability increased from 2008 to 2010. Furthermore, these data illustrate the influence of individual beaver dams that can cumulatively contribute to the downstream warming and/or cooling. Such hydrologic and temperature variability would be lost if only reach-scale measurements were collected. In the context of ecosystem impacts and potentially using beaver as a restoration tool, where habitat heterogeneity and increased system resilience is achieved through higher rates of biodiversity, we argue that quantifying the range and increase in variability may be far more important than measuring a minor and often inconsistent change in mean conditions.
This research was primarily funded by the Utah Water Research Laboratory and partially supported by National Science Foundation EPSCoR grant IIA 1208732 awarded to Utah State University as part of the State of Utah EPSCoR Research Infrastructure Improvement Award. Any opinions, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors would like to thank the Utah Division of Wildlife Resources for facilitating this research and the numerous field crew members for their help with data collection. In addition, the authors would like to thank to Megan Klaar, Michael Pollock, and an anonymous reviewer for their thoughtful comments that greatly improved this manuscript. Edited by: F. Pappenberger