ANALYSIS OF SUSPENDED SEDIMENT TRANSPORT DATA IN THE ST. CROIX RIVER

Emily Kushner, Macalester College, Department of Geology
Kelly MacGregor, Macalester College, Department of Geology
Elena Evans, Macalester College, Department of Geology
Daniel Hornbach, Macalester College, Department of Biology
Mark Hove, Macalester College, Department of Biology
Matthew Cox, Macalester College, Department of Biology
Ben Dickenson, Macalester College, Department of Biology

Hydrologic and sedimentologic conditions in the St. Croix River 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 collected by the Metropolitan Council near Stillwater show a decrease in suspended sediment concentration in the lower part of the river over the last 28 years. Other work (Hornbach and others) demonstrates a decrease in the grain size of bed sediment at Interstate Park since 1990. During this same time, a 96% decline in the juvenile mussel population was also recorded at Interstate. Both Interstate and Franconia are home to two federally endangered freshwater mussel species, the winged mapleleaf (Quadrula fragosa) and the Higgins Eye Pearlymussel (Lampsilis higginsii). We need to better understand the controls on sediment transport in order to understand the causes for this decline in the juvenile mussel populations and to evaluate future threats to these species.

In summer 2004, we began sampling suspended sediment (both surface water and near-bed) at various inshore and outshore locations along a 10-mile stretch of the St. Croix River (St. Croix Falls to Osceola). In summer 2005, we continued suspended sediment sampling at Wild River, Interstate Park, and Franconia. Most of our 2004 samples were collected during moderate discharges (4000-5000 cfs), while the 2005 samples were collected across a much greater range of discharges (1700-9500 cfs). These data will allow us to explore the controls on sediment transport by considering the relationship between suspended sediment concentration and instantaneous water discharge, mean daily discharge, water velocity, and other factors. Using these rating curves we can calculate total suspended sediment load. Sediment cores taken from the Lake St. Croix (Triplett and others) provide a comparison to long-term sedimentation rates. We hope to better understand the controls on sediment transport and deposition in the river over daily to decadal timescales, and to further explore the impact of sedimentological changes in the river on freshwater mussel populations.