Ada Hayden Lake, a 49.6 ha, gravel-pit lake containing 5.2 million m 3 of water, comprises the emergency water supply for Ames, Iowa. This study characterized and quantified groundwater/lake interaction for the assessment of water supply potential. Hydraulic head, delta18O, delta 2H, and 3H data collected from 23 piezometers show that Ada Hayden Lake is a flow-through lake. Geochemistry is characterized by high P concentrations (SRP=93.6 mug/L), denitrification, and methanogenesis. A 3-D, finite-difference groundwater flow model shows that groundwater contributes 85 percent of the lake's water budget and approximately 42 percent of the soluble reactive P load. Model-simulated pumping of 106 gal/d from the lake during drought induces flow from the South Skunk River and lowers lake stage 3.6 m after 30 days. Pumping 1000 gpm in the aquifer for a year produces a drawdown of 3.75 m, induces flow from the lake, and lowers lake stage about 0.09 m.