TNC Building Beaver Dams

View of a series of existing beaver dams downstream of beaver dam analogue restoration reach. Photo © The Nature Conservancy (Kristen Podolak)

Beavers can be highly destructive in the wrong environment, but are keystone species wherever they’ve been around for long enough to have designed their ecosystem. And in areas where for a number of reasons beaver populations have dwindled so much as to see deterioration in the habitat, The Nature Conservancy thinks that adding artificial dams can help restore the land by affecting sedimentation, creating floodplains, and storing water. Kristen Podolak, Rodd Kelsey, Sierra Harris, and Nathan Korb report:

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.

Read the rest of the article here.

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