Implications of water, sediment, and nutrient budgets for the restoration of a shallow, turbid lake in semiarid southeastern Oregon

First posted September 18, 2023 For additional information, contact: Director, Oregon Water Science CenterU.S. Geological Survey601 SW Second Avenue, Suite 1950Portland, 97204 Malheur Lake is the largest lake in endorheic Harney Basin southeastern Oregon. Since 1990s, Lake—which averages depths of about 1 meter—has been a degraded, turbid state lacking submergent and emergent vegetation. The goals this study were to identify major sources sediment nutrients determine importance managing for restoration. Discrete water samples analyzed nutrient (total phosphorus, total nitrogen, orthophosphate, nitrate+nitrite, ammonia) suspended-sediment concentrations, parameters including chlorophyll-a phytoplankton biomass measured samples. area fluctuated from minimum 3,300 hectares (ha) maximum 11,300 ha years 2019 2020. In year 2019, inflow tributaries created 1,400-hectare with low turbidity that persisted multiple months. Land-use practices diversions along affected hydrographs concentrations reaching lake. As increased, storage sediment-associated constituents column increased excess external loads because resuspension. 69 percent increase was attributed internal resuspension 31 loading. Sediment deposited as decreased, water-column decreased even positive loading continued. resuspension, deposition, suspended likely decreasing topographic heterogeneity Concentrations phosphorus orthophosphate are substantially higher than 1980s, eutrophic. Phytoplankton light limited 2019–20, restoration actions prioritize vegetation establishment would reduce bioavailable while increasing column.