ANALYSIS OF SEDIMENT TRANSPORT DATA AND CALCULATION OF HISTORICAL SEDIMENT LOAD, ST. CROIX RIVER, MN/WI

Sediment budgets in river networks are notoriously difficult to construct but can be extremely important for quantifying both short and long-term changes to fluvial environments. Adequate sediment supply is critical for in-channel, bar, and near-shore ecosystems (both aquatic and terrestrial), as well as for recreation and navigation purposes. Hydrologic and sedimentologic conditions in the St. Croix River, a tributary of the Mississippi that forms a partial border between WI and MN, play a significant role in the stability of native freshwater mussel populations. The transport of sediment controls overall geomorphology, riverbed composition, and water turbidity, all of which are important to mussel habitat. Data shows a decrease in suspended sediment concentration, a decrease in the grain size of bed sediment, and a 96% decline in the juvenile mussel population in the last decade below the only major dam in the river system. The St. Croix River is home to two federally endangered mussel species; we need to better understand the controls on sediment transport to understand the causes for this decline and to evaluate future threats to these species.

In conjunction with mussel habitat analysis, we have collected surface and near-bed suspended sediment, as well as bedload samples since 2004. Discharge in the St. Croix typically ranges from 1000-20000 cfs. Samples collected during a range of discharges (1700-10000 cfs) along with other suspended sediment transport data (1974-2003, USGS) will allow us to explore the controls on sediment transport which include discharge, water velocity, depth, shear stress, shear velocity, and stream power. Preliminary analyses show a positive correlation between suspended sediment concentration and discharges above 5000 cfs. Using these rating curves we calculate total sediment load over the past century. Further work will allow us to understand the controls on sediment transport and deposition over daily to decadal timescales, and to explore the impact of sedimentological changes on mussel populations.