Sediment Source Identification In A Northern Forested Watershed – Leveraging Sediment Budget and Sediment Fingerprinting Methods Toward Understanding Erosion In The Little Fork River, Minnesota

Watersheds dense with forest and wetland are not commonly thought of as settings that suffer from excess erosion; however, they can generate large amounts of sediment depending upon underlying geologic context and historical land use practices, such as timber harvest. The Little Fork River Basin in northern Minnesota is heavily eroding and has a disproportionate source of sediment and phosphorus to downstream waters while being densely forested, with extensive wetlands, mature forest, and managed timber lands. The Little Fork River has been designated as impaired due to excess turbidity and total suspended solids, and the Minnesota Pollution Control Agency (MPCA) is working to develop and implement total maximum daily load (TMDL) regulations to address these impairments. Successful TMDL development and implementation relies upon detailed information describing sediment sources. The U.S. Geological Survey is working in partnership with the MPCA, the University of Minnesota, and three local Soil and Water Conservation Districts to delineate the proportion of dominant sediment and sediment-bound phosphorus sources contributing to the Little Fork River and downstream waters using two Environmental Protection Agency certified methods: sediment fingerprinting and sediment budgets. Sediment fingerprinting uses the geochemistry of sediments to tie fluvial sediment to upland and near-channel sources within the landscape. Sediment budget methods use measurements of eroding features within the river corridor such as stream banks, valley sides, and ravines to estimate volumetric loading from those locations and extrapolates to the rest of the network. Collection of data in support of these sediment budget and sediment fingerprinting efforts was carried out during the 2021 and 2022 field seasons. Rapid Geomorphic Assessments (RGAs) were conducted in support of the sediment budget at 46 locations throughout the watershed, across scales from headwaters to ravines to mainstem channels, to determine the extent of erosion and deposition at representative locations along the channel network. The stream network was delineated to ensure all ravines of the scale sampled were represented. Data collected at the RGA locations were extrapolated to this detailed river network using characteristics such as channel slope, valley side slope, stream order, and parent material. Source sediment samples for geochemical fingerprinting were collected at more than 60 upland locations representing mature and recently harvested forest, agricultural fields, and roadways, and at 46 stream and near channel locations corresponding to the RGA data collection reaches. Fluvial sediments for source determination were collected from the streambed at the RGA sites, and fluvial suspended sediment was collected across two monitoring seasons at eight locations representing the Little Fork mainstem and tributaries to the Little Fork that contribute to sediment loading. An overview of the techniques, data collection, and progress toward completed sediment and sediment bound phosphorus source assessments will be presented.