DAM! IMPACTS OF BEAVER DAMS ON SURFACE AND GROUNDWATER QUALITY

The overall goals of this project are to identify and quantify key nutrient (e.g., N, P) removal processes associated with beaver dams, and estimate the potential impact of dams on watershed-scale nutrient loading within the agricultural Midwest. Long-term, this project aims to provide the Iowa Nutrient Reduction Strategy (INRS) Science Team with data that clarify the influence of the stream channel and processes associated with in-stream nutrient loss on watershed-scale nutrient loads. Project goals are especially pertinent in the context of the 17% discrepancy between estimated and observed nitrate loading, as discussed within the INRS. Beaver (Castor canadensis) activity (i.e., damming of streamflow) holds significant potential to impact: in-stream nutrient processing, through reduction of streamflow velocity and increase of water residence time within pools, trapping of sediment and organic material, raising of riparian groundwater tables, restoration of channel-floodplain connectivity. Thus, dams may represent a “no-cost in-stream conservation practice” that provides compound benefits beyond water quality and quantity, such as enhanced wildlife habitat and increased riparian vegetation diversity.

Preliminary results from the 2021 field season are promising, especially regarding stored sediments driving potential N-removal. A lab based, field sample derived N-removal assay has been developed and successfully piloted. We have a target to assess, characterize, and sample sediment from 30+ dam pools across the Des Moines Lobe (DML) landform in the 2022 season. Pool and reference reach sediment N-removal will be examined in the contexts of sediment type, organic matter content, stream/pool position, stream discharge, and a number of other factors. The results of these analyses will be mapped back to the pool area in order to estimate a removal rate (mg NO₃), per stream bed area (meters squared), per time (day). In parallel to the exploration of N-removal capabilities of beaver dams, their ability to trap P, is being examined. Bed sediments sampled for N-removal will also be sampled for total P, Bray 1 & 2 available P, particle size, and other analytes. This will be used to quantify the volume, mass, and seasonality of phosphorous impounded behind a range of dams, across the DML.