DOES LOW DISCHARGE FROM THE DAM AT ST CROIX FALLS DAMN THE MUSSELS THERE?

Dams affect many aspects of a river system simply by regulating discharge, which in turn affects sediment transport and water temperature. The adjustment of just a few variables of a river system can have drastic implications for the ecosystems present in the river. The St Croix river is an excellent system to study because it has a dam (located at St Croix Falls) and the river is protected under the National Wild and Scenic Rivers Act of 1968. In addition, there is already a wealth of data that researchers can tap into on the hydrological and biological characteristics of the river.

The dam regulates water levels in both daily and annual cycles, and at times the discharge of the river is low enough that large swaths of the river bed are subaerially exposed.. We characterized these periods as low discharge events (LDEs) and used historical data (1983-Present) from the USGS to characterize the frequency and magnitude of these events to infer what impact they had on aquatic life, especially the mussel population. Our preliminary findings indicate that there have been many more LDEs in winter months than during the summer in recent years. We have been in dialogue with officials from the DNR and at the dam about the history of regulations regarding minimum discharge in order to determine whether potentially harmful low water levels were permissible given the standards at the time. We are also examining the relationship between discharge, temperature fluctuation and suspended organic solid concentration taken from on site measurements.