Changes in the mussel density over time can be found here.

References:

Gunderson, L., R. Finley, H. Bourne and D. Lofton. 2015. Sediment Reduction Strategy for the Minnesota River Basin and South Metro Mississippi River: Establishing a foundation for local watershed planning to reach sediment TMDL goals. MN Pollution Control Agency. Document wq-iw4-02.

Bernard N. Lenz, B.N., D. M. Robertson, J. D. Fallon, and R. Ferrin. 2003. Nutrient and Suspended-Sediment Concentrations and Loads, and Benthic-Invertebrate Data for Tributaries to the St. Croix River, Wisconsin and Minnesota, 1997–99. U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 01–4162.