Lake Hefner is a terminal offset reservoir located in Oklahoma City and used as a water supply lake. The lake is eutrophic and customers frequently complain of tastes and odors in finished water. During 1980 to 1981, water and nutrient budgets were constructed to learn the causes of eutrophication of the lake. Most water and nutrients entered the lake via a 11.6 km canal from the North Canadian River. The loadings were regulated so that flow events in the canal coincided with releases of water from Canton Lake, which is located upstream on the North Canadian River, or when rain fell on its watershed. Discharge and nutrient content were monitored during these flow events. Linear regression of concentration of nutrients on discharge showed soluble reactive phosphorus decreased with increasing discharge rate in both North Canadian and Canton flow events. Ammonia, nitrate, and Kjeldahl nitrogen increased as discharge rate increased only in North Canadian flow events. Canal discharge did not have typical features of stream hydrographs. Therefore, canal discharge may not be usable as an infallible predictor of nutrient concentrations. Estimation of nutrient loading of lakes via canals may of necessity remain highly empirical.