Offset reservoirs deserve study because of their importance in water supply systems, yet little information is available about how these lakes process nutrients. To close this gap the water, nitrogen and phosphorus budgets were determined for an offset reservoir, Lake Hefner, Oklahoma. During 1981, about 87 percent of the water and 88 percent and 97 percent of the nitrogen and phosphorus entered the lake via a canal connecting the North Canadian River to the lake. The lake accumulated both nitrogen and phosphorus. The sedimentation coefficient (R) for phosphorus was 0.86 yr−1. Loading models developed for artificial lakes predicted roughly the same total phosphorus concentrations as were observed in the lake, although not precisely. Apparently, offset reservoirs are not special cases of artificial lakes. These conclusions were based upon a data set for only one year and do not reflect year-to-year variability. An empirical monitoring approach is suggested for management of offset reservoirs.