Integrating Geomorphologic Field Assessment and Watershed Modeling for a Turbidity TMDL

It is important to distinguish watershed loading from upland and near stream sediment sources in TMDLs addressing sediment and/ or turbidity impairments. Upland sources from roads, construction, development and other land use disturbances contribute sediment to rivers. Near stream sediment sources, such as stream channel and bank erosion can contribute significant quantities of sediment and can be difficult to quantify. This paper describes a methodology to quantify these sources through the integration of a physical channel assessment and watershed computer modeling adopted for a northeastern Minnesota watershed. In support of a TMDL to address turbidity impairment, computer models (WEPP 2005.6 and FLUX) and a geomorphologic assessment were conducted to identify and estimate delivered loads from sediment sources in the watershed. The computer modeling, physical channel assessment, and the techniques used to integrate the two studies are presented in this paper. The integration of modeling and field assessment was critical because each quantified sources of sediment at different spatial and temporal scales. Since stream bank erosion may contribute a large portion of the sediment load, accurate estimates of both upland and near stream sources are important for a scientifically defensible TMDL.