EWN Publications

Long-term nitrate-nitrogen reductions in a large flood control reservoir

Keith E. Schilling, Elliot Anderson, Matthew T. Streeter, and Chuck Theiling
August 7, 2023

About This Publication

Flood-control reservoirs are common in the U.S. and several are located in highly agricultural regions of the Midwest. In this study, a mass balance of annual and seasonal NO3-N concentration and load reductions was calculated for Lake Red Rock, a large flood-control reservoir located on the Des Moines River downstream of the City of Des Moines, Iowa. Utilizing rare long-term upstream and downstream NO3-N monitoring records extending back in time 42 years and combining this with estimates from tributary inputs, Lake Red Rock reservoir was found to remove an average of 7,379 Mg NO3-N per year, representing 12.4 % of the NO3-N inputs to the reservoir. Annual variations in NO3-N loss rate varied considerably (-2.9 % to 41.3 %) and were significantly related to annual reservoir water retention time. Combining study results with literature data, annual NO3-N reduction in Midwestern flood-control reservoirs was logarithmically related to average annual water retention time. Although flood-control reservoirs are designed primarily for flood protection, manipulating the reservoirs to achieve longer water retention times, particularly during periods of high NO3-N concentrations and loads, could be a strategy to alleviate impairments at downstream water suppliers and help state and federal agencies meet NO3-N reduction goals.

Research Projects

The practice of managing reservoir and navigation pools to increase wetland habitat value while maintaining safe navigation and managing flood risk is referred to as Environmental Pool Management (EPM). EPM can be a c...