BDAs in the stream
The project design mimics the work of beavers by using beaver dam analogues, or BDAs, in the stream. BDAs are lines of wooden posts that are woven with branches to slow and redirect stream flows. BDAs help to both release and capture sediment, building the stream bed up, raising the water table, and increasing the complexity of the system.
Photo: BDA 30, the upstream-most beaver dam analogue at Triple Creek. This photo shows how BDAs work to slow and back up water. At this location, spring flows connect the creek with the floodplain, making the wetland wet again!
BDAs are designed to do different kinds of work in the stream. Channel-spanning BDAs slow the water down, back water up, and capture sediment. Deflector dams push the water into the bank, pulling sediment out of the bank and into the creek. When used in combination, BDAs build the elevation of the creek bed, increase creek sinuosity, reduce the slope (steepness) of the creek, and allow varied substrate – from rocks and gravels to fine silt – to fall out of suspension. By slowing water down and increasing the complexity of the hydrologic system, BDAs allow water to soak into the spongy wetland soil, they develop varied habitat for a diversity of species, and ultimately help to support a resilient ecosystem.
The slideshow below shows how the creek changes in response to BDAs as well as how we manage BDAs in response to changes in the creek.
Planting Near the Stream
A diverse and robust riparian buffer zone will not only improve native vegetation but will encourage beavers to recolonize the area. Ultimately we hope that nature’s furry, flat-tailed engineers will take over management and maintenance of the site.
Volunteers of all ages have helped plant willow, dogwood, cottonwood, chokecherry, elderberry, Douglas-fir, spruce, snowberry, spirea, and other species of trees and shrubs. As these plants grow, they will provide shade, healthy wetland function, and food and habitat for wildlife.
The slideshow below shows how we suppress invasive reed canary grass and thistle, plant native species, protect plantings from browsing, and maintain planting plots by weeding and watering until the plants are well established.
Virtual Field Guide to Beaver Dam Analogs
Bringing beavers back
Beavers have already shown increased interest in the project site, starting just a few days after construction began in 2016. Beavers even wove a post line themselves in 2017, before the project team could get to it!