The Nature Conservancy is working like a beaver (Castor canadensis) by mimicking beaver dam building to restore streams and floodplain habitat in Montana and California.
No kidding. Last year we built twelve instream structures that look and act like beaver dams on two streams in Montana and we plan to build six more in a small creek in Childs Meadow, California this fall.
Why try to act like beavers? Beavers are not a panacea and can be a nuisance when they block water diversions or chew down people’s favorite trees, which is why they have been persistently trapped and killed or relocated in many areas across North America.
However, when beavers create dams and wetlands they reconnect floodplains, increase water storage, and may provide habitat for threatened species. In Montana and California we will monitor how constructed “beaver dam analogs” might be an effective tool for stream restoration.
The goal of beaver dam analogues (BDAs) is to provide short-term structural complexity that facilitates dynamic physical and biological processes. The structures go by many names: beaver mimicry structures, deformable grade structures, beaver dam analogues, beaver dam support structures, or post-assisted willow structures. The diversity of names relates to the specific design purpose and whether using the term “dam” or “beaver” is acceptable to the local community and permitting agencies.
BDAs are made of vertical wooden posts pounded into the streambed across a channel with willow branches woven between the posts. Soil, small rocks, and organic material are packed on the upstream end of the structure to make it less porous and to pond more water.
While the beaver would not pound wooden posts into the streambed, it saves us humans’ time and helps guarantee that the structures will withstand high flows. Willows are also lined parallel to the stream on the downstream end to reduce scouring that might create a deep pool and undermine the BDA. Another option for building BDAs is installing vertical wooden posts pounded into an existing beaver dam to help reinforce it, which gives the beavers a hand in maintaining their own dams.
In Montana, The Nature Conservancy partnered with Montana State University to understand how BDAs used to restore incised channels may improve stream and riparian habitat, improve water quality (e.g. reduce excess sediment and nutrient loads), reduce stream temperatures in the summer, and improve natural seasonal storage in shallow alluvial aquifers.
The effects of BDAs on natural water storage have yet to be tested directly, and the specific hydrologic mechanisms that would promote higher and cooler late summer flows remain poorly understood. Furthermore, the potential for loss of water due to higher evapotranspiration rates offsetting the benefits of enhanced natural storage has not been investigated. These key gaps in understanding prevent a quantitative basis for the use of BDAs as a viable strategy for drought resiliency planning and management of cold-water fisheries.
In California, The Nature Conservancy partnered with the University of California, Davis, U.S. Forest Service-Pacific Southwest Research Station, and Point Blue Conservation Science to study BDAs and their effect on water storage, threatened species, and carbon sequestration.
The funding for the BDA restoration project is from the Greenhouse Gas Reduction Fund, a Cap and Trade Program, aimed to sequester carbon in California. The hypothesis of the study is that livestock grazing limits beaver populations from building dams because there are fewer willows along the stream. Beavers need willows for food and dam building.
By excluding cattle from the stream, planting willows, and building BDAs, we expect the restoration actions will sequester carbon in above and below ground vegetation and soils, as well as improve groundwater storage and habitat for threatened species occurring in the meadow (Willow flycatcher and Cascades frog). We are comparing the impacts of BDAs directly to conditions in the meadow downstream where beavers are actively building dams.
Beaver-based restoration could be a lot cheaper than current stream restoration approaches that involve realigning or redesigning stream channels using heavy earth moving equipment. Channel-spanning structures similar to BDAs have been added to streams for more than one hundred years to address channel incision, when the stream cuts into its bed and the channel becomes narrower and deeper. These often involve rock check-dams, rock vortex weirs, rocks in wire cages, log steps, log jams, or plugging the stream with earthen dams.
Similar to BDAs, all of these structures serve to reconnect the stream with its floodplain in order to increase the frequency that streamflows overtop banks, re-water the floodplain, and dissipate stream energy. Floodplain connectivity is important because it reduces risks to communities, infrastructure, and erodible banks downstream. Restoring river-floodplain dynamics also creates complex aquatic and wetland habitats that are some of the most productive and diverse in the region.
The differences with BDAs compared to the other channel-spanning structures used to restore streams are that: 1) BDAs are cheaper because there is no heavy equipment used; 2) ideally the beavers adopt and maintain them and their labor time is free, and 3) they are not designed to be permanent structures on the landscape and they will change over time, existing and disappearing as they provide or lose function for beaver colonies, which we think will more closely mimic the naturally dynamic nature of these ecosystems.
This idea is not new though. Michael Pollock and Chris Jordan from the National Oceanographic and Atmospheric Institute along with Nick Bouwes and Joseph Wheaton from Utah State University, Carol Volk from South Fork Research, Inc., and Nicholas Weber from Eco Logical Research Inc. pioneered the idea of BDAs in 2009.
They constructed more than 85 BDAs of various design to reduce channel incision, facilitate sediment accumulation to rebuild streambeds, and restore habitat for steelhead (Oncorhynchus mykiss). They found that compared to a nearby unrestored stream, the steelhead survival and abundance following restoration with BDAs in Bridge Creek was higher and so was the groundwater level, providing some of the first evidence for the multiple benefits of this approach.
BDAs are only appropriate in specific river settings where the channel gradient is low and where the channel is not confined to a steep canyon. They may be suitable for the mainstem or a tributary channel, depending on the average and seasonal flow in the stream.
BDAs are best suited for locations where existing beaver populations are nearby. Beaver populations may be limited from an incised section of stream because their dams tend to blow out during high flows as was the case in Bridge Creek, Oregon where the first BDAs were designed and monitored. In these cases, BDAs may facilitate slowing water down so that beavers can reestablish themselves into the system.
Any stream channel alteration involves complex river processes and should only be undertaken with careful consultation with teams of experienced hydrologists, geomorphologists, ecologists, and restoration specialists. BDAs, like other in-channel activities, require at a minimum instream permitting and environmental review.
We are excited about the possibility of BDAs to be a process-based and cost-effective stream restoration approach. We will share results from the Montana and California studies over the next five years on the impacts of BDAs on water storage and quality, carbon sequestration, and threatened species abundance and reproductive success. For more information on beavers as restoration agents and BDAs please see The Beaver Restoration Guidebook.