Beaver Dam Analogs (BDAs) alter carbon pools and fluxes in intermountain headwater streams

The use of Beaver Dam Analogs (BDAs) is a widely adopted, low-cost stream restoration technique that mimics the structure and function of natural beaver dams. Analogous to beaver dams, BDAs restore the stream’s connectivity with its floodplain by increasing water residence time and facilitating sediment retention in the stream channel. Greater connectivity afforded by BDAs is expected to drive changes in carbon pools and fluxes in ways similar to those in active beaver meadows where beaver engineering causes landscape-scale C storage but also releases the greenhouse gasses CO2 and CH4 to the atmosphere. It is assumed that carbon storage in BDA-restored streams will occur in the stream channel behind the structures and in riparian soils while simultaneously emitting greenhouse gases from the inundated soils. Although our research indicates that there are strong patterns that show four-fold increases in particulate organic carbon (POC) pools in BDA restored reaches, there is a lack of clear BDA effects on soil carbon and soil greenhouse gas fluxes. However, we expect that such differences may emerge as time since restoration increases. To examine the impacts of BDAs on the carbon economy of restored streams, we measured carbon pools and fluxes in BDA-restored and unrestored stream reaches in three headwater streams in western Montana. During the summer of 2021, two years after restoration, we measured instream and riparian carbon pools and the release of CO2 and CH4 from riparian soils. To quantify carbon pools, we sampled accumulated sediments within the stream channel and collected riparian soil cores along each stream. To assess carbon gas, we compared how elevated groundwater and surface water levels alter decomposition in soils by measuring the loss of both CO2 and CH4 to the atmosphere. Despite the focus of past research on the effects of BDAs on stream hydrology and fish habitat, there has been little research examining their influence on carbon pools and fluxes. Though BDAs have the ability to accelerate the recovery of degraded streams by maintaining water supply, elevated groundwater and surface water levels and increased organic matter pools have the potential to shift soil gas fluxes from CO2 to CH4, which has a global warming potential that is 84-Fold higher than CO2 on a 20-year timescale. Thus the positive effects of BDAs may in part be offset by increased greenhouse gas production. This study showed increased C storage but did not find strong evidence of increased CH4 emissions, suggesting that in the short term, the benefits of BDAs are positive at the ecosystem level. Further study is necessary to see what the longer-term impacts are likely to be as C accumulates