Quantifying the Impact of Iowa's Flood‐Mitigation Reservoirs on Sediment and Nutrient Loss

Flood‐mitigation reservoirs have long been known to impact pollutant transport by retaining or removing incoming sediment and nutrients. However, historical reductions in these systems have rarely been well quantified. In this study, we used water quality data to estimate inputs and outputs of total suspended solids (TSS), two phosphorus (P) forms, and three nitrogen (N) forms in three Iowa reservoirs (Coralville, Red Rock, and Saylorville). We also explored the influence of reservoir residence times on removal rates. Annual residence times were largely consistent across the basins, ranging from roughly 6 to 100 days (mean of 19 days). Between 2001 to 2023, most TSS (~ 80%) entering the reservoirs was retained. This sedimentation corresponded to average volume losses in the reservoirs' normal storage pools of 0.37%–0.85%/year. About 40% of P and 12% of N were likewise reduced—driven mainly by decreases in particulate P and nitrate. Residence time appeared unrelated to removal rates of TSS and particulate nutrient forms, but longer residence times coincided with increased nitrate loss. Reservoir impact on statewide nutrient export was significant, with loads in Iowa's major rivers being reduced by 9.8% (for P) and 4.7% (for N) due to reservoir capture. These findings suggest that reservoir operators may be able to facilitate further nitrate removal by lengthening storage durations without incurring additional sedimentation or generating other nutrient forms.