Surface water from the Des Moines River is impaired by nitrate-nitrogen for drinking water use. Saylorville Reservoir is a 24.1 km2 impoundment of the Des Moines River located approximately 10 km north of the City of Des Moines. Monthly mean nitrate concentration data collected upstream and downstream of the reservoir for a 30-year period (1977-2006) are analyzed in this study. Our objective is to improve understanding of the reservoir effects on river nitrate concentrations. We hypothesize that monthly water quality downstream of the reservoir depends on the current monthly upstream water quality and its past lags. The dynamic relation is studied via a transfer function model that is shaped by reservoir characteristics such as surface area, volume, and discharge. Research results may used to better manage Saylorville Reservoir to mitigate nitrate concentrations in downstream receiving waters and forecast potential impairment to the Des Moines Water Works.