BEDLOAD SEDIMENT TRANSPORT IN THE ST. CROIX RIVER
Elena Evans, Macalester College, Department of Geology
Kelly MacGregor, Macalester College, Department of Geology
Daniel Hornbach, Department of Biology
Mark Hove, Department of Biology
Emily Kushner, Macalester College, Department of Geology
Matthew Cox, Department of Biology
Ben Dickenson, Department of Biology
Rivers shape the landscape through the transport of both water and sediment. A long term study of the middle and lower St. Croix River by Hornbach and others has examined the relationship between native mussel populations and bed sediment grain size. This work suggests that bed sediment distribution, stability and transport are likely critical to the health and longevity of mussel communities.
During the summer of 2005, we initiated the collection of bed sediment at several locations on the St. Croix River both above and below the St. Croix Falls Dam. Bed sediment transport depends on shear stress conditions in the river (which in turn is controlled by water velocity, river gradient, and water depth) as well as the grain size distribution of particles resting on the bed. We made measurements of bedload transport across a wide range of water discharges during this time with a Helley-Smith sampler. The associated rating curves for different parts of the river will be critical for predictions of sediment transport and for consideration of the effect of bedload on mussel habitats. Earlier work suggests that while bed sediment grain size is relatively unchanged, below the St Croix Falls dam a fining trend unlike the other study areas is present. Further analysis of our bedload transport data (which is notoriously difficult to collect) will allow us to quantify changes to the physical fluvial environment over time.
