NEAR BED SEDIMENT TRANSPORT AS AN INDICATOR OF SUBSTRATE STABILITY AND SUITABLE MUSSEL HABITATS IN THE ST. CROIX RIVER

Leah Ritz, Kelly R. MacGregor, Mark C. Hove and Daniel J. Hornbach
Departments of Geology, Environmental Studies and Biology, Macalester College

The St. Croix River provides a pristine habitat for several mussel species including some federally endangered and threatened species. Suitable mussel habitats within the St. Croix are characterized by environments with low shear and frictional stresses allowing the buildup of stable, supportive substrate while providing sufficient food and other materials like oxygen and calcium. One objective of this study was to determine tsubstrate stability and how it relates to mussel density at Wild River and Interstate Park. We measured near-bed sediment transport using a Helley-Smith bedload sampler at Wild River, a sandy habitat upstream of the St. Croix Falls dam and Interstate Park, a rockier habitat downstream of the dam. A significant relationship was found between water discharge and bed sediment transport at both high (>5000 cfs) and low flows. There was a difference in average grain size between Wild River and Interstate, and a difference in the relationship between water discharge and bed sediment flux at each location. Furthermore, there was a significant difference in the number of mussels found at Wild River and Interstate. Further measurements at higher water discharges (near peak annual flows) will be required to refine our understanding of how shear stress relates to sediment transport and ultimately to preferred mussel habitat.