Artificial Water-Level Management and Relation to Water Quality and the Aquatic Ecosystem
Altered water-level management has numerous potential effects on lake ecosystems. The U.S. Geological Survey and National Park Service have collaborated on several studies to evaluate water-level changes in Voyageurs National Park lakes and how they are related to various water-quality and ecological effects. The International Joint Commission set new rules governing dam operation for this system of lakes in 2000 and these collaborative studies have provided useful information for the International Joint Commission’s assessment of whether or not to maintain these new rules.
Rule curves, defined as the maximum and minimum allowable water levels throughout the year, have been established for Namakan Reservoir and Rainy Lake on the USA-Canadian border. One study (2001-2003) showed early indication of improvement in chlorophyll-a concentration and Secchi depth in Kabetogama Lake and Black Bay, two of the most eutrophic water bodies in the park. Substantial improvements were not seen for total phosphorus. A follow-up study (2008-2009) confirmed these results and suggested a possible link between frequent recirculation and internal recycling of phosphorus.
Additional studies have evaluated trophic state, flow direction, fish mercury concentrations, and cyanobacterial harmful algal blooms. The results of these studies will be presented followed by a discussion of whether the rule curves appear to have benefitted the ecosystem.