Sediment response to stream restoration in an agricultural watershed

Despite widespread implementation of stream restoration there is limited research of its effectiveness in impaired agricultural settings. To quantify the effects of stream restoration on sediment load, we investigated erosion and deposition along 9 stream reach sites (4 restored, 4 unrestored, and 1 downstream) in Turtle Creek, Pennsylvania, United States - a representative agricultural subbasin of the Chesapeake Bay. These stream restoration projects had the goal of reducing sediment load primarily by installing log structures and regrading streambanks in order to stabilize streambanks and reconnect floodplains. Floodplain and streambank sediment fluxes were quantified over a 3-year period through repeated measurement of clay pads and pins and measurement of sediment bulk density. Uncrewed aerial imagery was collected at 3 of the sites, then processed using structure from motion (SfM) to create high-resolution digital elevation models. Terrain analysis enabled three-dimensional understanding of the generality of the specific field flux measurement locations to the larger stream reach. Restored sites on average had deposition on streambanks compared to streambank erosion along unrestored sites, and approximately two times greater floodplain deposition than unrestored sites. Overall, restored stream had an average seven-fold increase in the net balance (sum of streambank and floodplain) of sediment flux, a 81.2 kg/m/yr increase in retention, compared to unrestored stream. The measured larger streambed particle size distribution (D₅₀) in restored streams also suggests additional improvements to instream habitat quality. Terrain analysis confirmed that the field-based measurements were representative of the larger reach. These results indicate that stream restoration in agricultural watersheds using less-engineered designs can reduce sediment loading and improve stream habitat and health goals.