Posted on Friday, June 02, 2017 (PST)
A 10-year study of the influence of beaver dams – some artificially added to the stream during the study – on stream water temperature found that the dams increase surface water storage and encourage cool groundwater flow. The results are a cooler stream at a time when typically high summer temperatures stress threatened juvenile steelhead.
The study’s authors tracked beaver dams and recorded changes in stream temperature along 34 kilometers (21 miles) of Bridge Creek, a tributary of North Central Oregon’s John Day River. The river is host to the Middle Columbia River steelhead, a species listed as threatened under the federal Endangered Species Act.
Summer temperatures in some areas of the creek will often exceed 25 degrees Celsius (77 degrees Fahrenheit), the upper limit of a steelhead smolt’s tolerance. It’s the cooling provided by beaver dams that can provide the fish with cool sanctuaries.
“Our observations reveal several mechanisms by which beaver dam development may influence stream temperature regimes; including longitudinal buffering of diel summer temperature extrema at the reach scale due to increased surface water storage, and creation of cool-water channel scale temperature refugia through enhanced groundwater-surface water connectivity,” the study says, which was published May 17, 2017 in the journal PLOS one.
According to the authors, past studies, as well as assumptions about beaver dams, had thought the dams resulted in warmer instream temperatures, but this longer term and more expansive study of a watershed instead shows that the dams have a more positive impact on temperature.
“Being that we tracked stream temperatures and beaver dams throughout a watershed over a 10-year period, our study does represent an expansive research project,” according to information provided by Nicholas Weber of Eco-Logical Research, Inc. in Bend, Oregon.
“As scientists have come to better understand the many ways that beaver impact stream processes, such as how dams increase surface water storage that may be less susceptible to heating during dry summer months, and how dams can promote connectivity with cool groundwater flow, more research was warranted to really understand the thermal implications of beaver complex establishment,” Weber continued. “We think this project does a great job of demonstrating how proliferation of beaver dams may impact stream temperatures over the long-term, and does so in an arid stream environment that is representative of the habitat used by trout and salmon populations throughout the western U.S.”
Weber’s co-authors in the study “Alteration of stream temperature by natural and artificial beaver dams” (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0176313), are Nicolaas Bouwes, Eco-logical Research, Inc. and Department of Watershed Sciences, Utah State University in Logan, Utah; Michael Pollock and Chris Jordan, NOAA Fisheries, Northwest Fisheries Science Center in Seattle; Carol Volk, South Fork Research in Seattle; Joseph Wheaton, Department of Watershed Sciences, Utah State University; and Gus Wathen and Jacob Wirtz, Eco-Logical Research, Inc.
The research was funded by NOAA Fisheries through the Bonneville Power Administration.
“Specifically, this research was set up to determine just what do we get back for our restoration dollar in terms of increasing fish populations, and what is the most cost-effective way to get there?” Weber said. “As it turns out, encouraging beavers to do the work they once did throughout the stream systems of North America works pretty well.”
Restoration work in Bridge Creek included the addition of what the authors call “beaver dam analogs” or a series of piles driven into the stream bed that are designed to mimic the function of natural beaver dams, giving beavers a head start on building their own dams.
This differs from other stream restoration projects, according to Weber. “Our research may not be very different from many projects, but our restoration approach of partnering with beaver is rather unique.”
Traditional stream restoration work will often employ heavy equipment in stream channels, a practice that results in disturbance and high costs.
On the other hand, the artificial beaver dams can be constructed by hand, leaving the stream and surrounding riparian vegetation intact. Once ponds have been created by artificial beaver dams, beaver are much more likely to occupy the area and take over the hard work. This results in an effective restoration approach without the initial disturbance and high-cost of more traditional restoration practices, according to Weber.
During the study and stream restoration efforts, the number of beaver dams in the study area increased significantly (the study says by “an order of magnitude”) and an additional 4 km of creek received the analogue dams. In most cases these analogue dams were “enhanced by active maintenance by beaver,” the study says.
Differences in stream temperature were most pronounced during the summer. Prior to the study and restoration daily maximum summer stream temperature lower in the study area (river km 6.16) was 5.06 degrees C higher than upstream at river km 32.39. After the stream improvements that temperature difference was halved to 2.5 degrees C.
“…our observations demonstrate that beaver pond creation has the potential to alter stream temperature regimes to the benefit of temperature sensitive stream biota such as the population of steelhead that reside within our study area,” the study says.
And, it could be one tool to counter climate change, which is expected to reduce the amount of habitat available to sensitive fish species.
“Faced with the threat of increasing stream temperatures, effective restoration practices that can mitigate for human impacts and climate change are essential to the conservation of these economically and intrinsically valuable fish species,” Weber said. “Installation of beaver dam analogs and the natural dam building activity of beavers may offer an effective and cost-efficient way to offset these impacts and increase the likelihood that these species will persist for future generations.”
Because of their severe impacts to riparian willows and willow wetlands (see photo below), cattle grazing as practiced on Klamath Mountains national forests precludes beaver restoration, included in the headwater basins where we could use beaver to counteract decades of cattle-trampling-induced hydromodification which has damaged baseflows in streams below.
Cattle grazing in our headwater basins has much more of a negative impact on salmon streams and habitat that you may have imagined. Here’s the link to hydrologist Jonathan Rhodes report on the Big Meadows Grazing Allotment which discusses these impacts: https://www.dropbox.com/s/j3heofz33bboh9n/RhodesRpt_KNF-BMA_2-18-16.pdf?dl=0