Stream Corridor Sources of Suspended Sediment and Sediment-Bound Phosphorus From An Urban Tributary To The Great Lakes

Potential sources of suspended sediment and sediment-bound phosphorus (P) were studied in the Kinnickinnic River (51 km2), a heavily urbanized tributary to Lake Michigan (about 84 percent urban land cover) in Milwaukee, Wis. The river is comprised of mainly concrete lined channels with some unlined eroding reaches. Total suspended solids is a pollutant in many Wisconsin streams, including the Kinnickinnic River; other contaminants, including total phosphorus, have degrading impacts on habitat, biological community, and dissolved oxygen. From September 2019 to October 2020 a stream sediment budget and sediment fingerprinting integrated study was conducted to quantify upland and stream corridor sources of suspended sediment and sediment-bound P using the Sediment Source Assessment Tool (SedSAT) methods with a suite of trace elements. Passive suspended sediment samplers were installed at three sites and sampled monthly. Soft, fine-grained streambed sediment was collected if present at 18 rapid geomorphic assessment (RGA) sites. An inventory of bank erosion and soft-sediment deposition was also done at each of the 18 RGA sites, which were selected to represent a range of stream sizes and geomorphic conditions. Sources of suspended sediment varied with streamflow; the primary source derived from roads sampled from residential areas followed by streambank erosion. Industrial areas contributed a small proportion of suspended sediment at the basin outlet during the fall and winter, whereas roads in parks contributed a substantial proportion at a mid-basin monitoring location in an unlined section downstream from a park. The dominant sources of fine-grained soft streambed sediment throughout the basin were eroding streambanks and residential areas, similar to suspended sediment, with natural and industrial areas contributing small proportions depending on locality. In contrast with previous studies in agricultural and mixed-use basins, the highly urbanized Kinnickinnic River had limited storage of soft sediment, likely due to the hydrologically flashy system. However, eroding streambanks consistently make up a substantial portion of suspended and soft streambed sediment across this tributary to the Great Lakes.

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