WATERSHED-SCALE MANAGEMENT FOR FINE SEDIMENT LOADING: BALANCING ALTERNATIVE RESTORATION SCENARIOS FROM BANK STABILIZATION TO WATER RETENTION (Invited Presentation)

Hydrologic regimes in the upper Midwest have changed over the past century through a combination of climate change and extensive land modification. In southern Minnesota, hydrologic shifts towards greater discharge have exacerbated erosion along deeply-incised channel corridors. Managing fine sediment loading involves prioritizing not only restoration sites, but entire restoration approaches, ranging from bank stabilization to water retention to more traditional conservation practices. To evaluate different suites of management strategies to reduce sediment loading in the 9,200 km² Greater Blue Earth River (GBER) basin, we developed the management option simulation model (MOSM). MOSM is a decision analysis model centered on a reduced complexity model that simulates the movement of water and sediment through a watershed. MOSM was constructed via input from a stakeholder group and is designed to be run in real-time enabling rapid scenario testing during stakeholder meetings.

MOSM is based upon a detailed sediment budget constrained by total suspended solids loads at 9 gages in the GBER basin. Hydrologic inputs derived from a SWAT model are routed downstream via Muskingum-Cunge routing. Field-scale erosion rates come from USLE data modified via a sediment delivery ratio that accounts for relief and distance from channel. Management options can act either on direct erosion rates or on sediment delivery ratios, depending on the action. For example, reduced tillage may lower the field erosion rate while installation of grassed waterways will reduce the sediment delivery ratio. Erosion rates of near-channel sediment sources (bluff, bank, ravine) are set by the sediment budget. Bluff and bank erosion can be reduced via direct action (i.e. bluff stabilization) or through hydrologic management in the uplands. By combining management option choices with costs and efficiencies, scenario evaluations can be run that return both predicted sediment load reductions and total costs at a watershed-scale. Initial results show that hydrologic management may be an effective method to reduce sediment loading downstream in the GBER basin. Interactions with stakeholders highlight the importance of having a diverse group when developing a consensus strategy for sediment source reduction.