Human-built 'beaver dams' restore streams
Beavers are a critical asset in Washington, assuring that healthy riparian zones are maintained, especially in the dry climate east of the Cascades. Beaver dams and ponds support native vegetation and wetlands along streams, trap sediment, recharge groundwater, and improve water quality.
Over the last two centuries, these benefits have been lost in many watersheds, following human development, beaver removal, channel deepening, and other impacts.
In 2015, the Okanogan Highland Alliance (OHA) was awarded a grant to restore a reach of Myers Creek, through Ecology’s Water Quality Financial Assistance Program. In the 1990s, Myers Creek was damaged in a major rain-on-snow event, which caused unusually high stream flows, deepening the creek, leaving vertical cut banks, and draining nearby wetlands.
Where beaver ponds had once provided grade control and covered large areas of the floodplain, the now-drier soils began to favor invasive plant species. The understory is now dominated by reed canarygrass (Phalaris arundinacea) and Canada thistle (Cirsium arvense), further suppressing growth of native sedge and forb species. The only remaining native riparian species still easily found in the project area is gray alder (Alnus incana), with a few isolated willow (Salix spp.) plants.
The biggest challenges at this site were the lack of vegetation needed by beaver for food and dam-building, and a straightened, incised channel that now flowed with too much force in the spring to allow beaver dams to take hold.
OHA, in partnership with the US Fish and Wildlife Service and Trout Unlimited, needed to find a cost effective tool and strategy that would jumpstart ecological recovery at this site and restore it to a condition where beaver could recolonize and maintain riparian function again. That tool was Beaver Dam Analogues, also called BDAs.
Developed by Michael Pollock (NOAA) and colleagues, BDAs offer a low-cost, simple, and easily scalable technique for mimicking beaver dams. They reduce stream velocity, induce lateral channel migration, and cause rapid aggradation of the streambed, which reconnects the floodplain so it can once again support riparian vegetation.
The human-built beaver dams are constructed by driving 3 to 6 inch diameter posts into the stream substrate. Then a hand-woven matrix of various tree species, like dogwood and Douglas-fir, is interlaced between posts to provide the resistance to retain water and trap sediment.
BDAs can be constructed as either channel spanning or deflector dams. Channel spanning dams slow the water down and capture it, trapping sediment and raising the streambed (aggradation). Deflector dams leave an aperture for the water to flow around, recruiting sediment from the banks for downstream capture and increasing channel sinuosity.
However, unlike natural beaver dams, the analogue dams must be maintained if they are to continue functioning year after year. This maintenance is needed until vegetation starts to return (or planted vegetation becomes established) and the site can provide enough food and dam material to entice beaver back to the system to take over maintenance of the site.
After extensive planning and permitting efforts, OHA and its collaborative team installed 26 of the dams along 1,900 feet of Myers Creek in the fall of 2016. They also installed staff gauges at many of the sites to measure both sediment deposit and to provide a reference for photo points over time. Water level data loggers were installed in the stream and in groundwater monitoring wells to measure changes in water surface elevation.
OHA also launched aggressive reed canarygrass management and native planting to help kick-start vegetation recovery. Fourteen planting plots and numerous caged plants were installed throughout the project area. In addition to providing shade, diversity, and stability, it is hoped that these plants will provide a more desirable, extensive food and dam construction source for beaver.
The timing of the construction proved fortuitous. When the spring of 2017 again brought exceptionally high flows to the watershed, this stream reach now had enough roughness to force dramatic, restorative geomorphic changes. Instead of further incising the channel, flows overtopped, flanked, and were deflected by the 26 new dams, which helped to widen the incision trench, increase sinuosity, and aggrade the streambed with freshly recruited sediment.
Sediment from the banks and the upper watershed was deposited up to nearly 5 feet in some areas, raising the channel closer to the historic floodplain. After these flow events, an annual survey showed an increase depth of water of over 11 percent in the first season following BDA construction.
Along with the restorative benefits to Myers Creek, this project has also become an important learning site in improving design and function and a training ground for restoration practitioners from around the Northwest wanting to learn how to install BDAs. Volunteers have been a crucial part of the project, contributing over 1,100 hours of service to date, assisting with BDA weaving, riparian planting, monitoring, and education.
One practitioner, Aaron Rosenblum with the Foster Creek Conservation District, found the opportunity to work on the site particularly insightful.
“My two visits to Triple Creek, one pre-peak flow and one post, illustrated to me the utility of BDAs as a restoration tool. The site was completely changed in just a matter of months: sinuosity was increasing before my eyes and several feet of additional water was being stored behind each structure. In Foster Creek, a system with streams that have been historically straightened and eroded, structures like BDAs could greatly help to restore the natural function and condition of the creek and improve water quality.”
In Triple Creek, project managers were surprised to see that after largely abandoning the site almost twenty years ago, beavers reoccupied the reach just days after BDA construction began in 2016. Their presence, while crucial for long-term maintenance, was initially challenging in that they eagerly ate the desirable willow, which was initially woven into the structures. This necessitated supplemental weaving to maintain the hydraulic benefits of each BDA, while also providing additional willows for the beaver to eat to relieve pressure on the repairs. In 2017, the team adapted their weave material to consist of less-tasty conifer boughs with red osier dogwood, which solved the issue.
Additionally, in the summer of 2017, beavers built a dam on a yet-unwoven BDA, instilling confidence that the long-term goal of beaver-based sustainability is achievable. Beavers also added to the weave material on another BDA where they sealed the dam at the base and provided desirable toe protection. In the latter part of 2017, beavers also built a starter dam just downstream of the downstream-most BDA, adding to project effectiveness much sooner than anticipated.
The long-term vision for BDA projects is that beaver will once again maintain dams to provide local grade control, floodplain connection, and wetland habitats to support a diverse flora and fauna. Sometimes partners like the Okanogan Highlands Alliance, the Washington Department of Ecology, along with many others, just have to help give them the boost they need.
Okanogan Highlands Alliance is a grassroots conservation organization based in Tonasket, Washington. Visit their website to learn more about the Triple Creek Project.
Visit NOAAs website to learn more about how beaver dams, and beaver dam analogues, help restore stream habitat.