The increase in dissolved gas concentration downstream of hydraulic structures such as dam spillways can be harmful to most fish species. Such increases have been noted at numerous hydroelectric dams on the Columbia and Snake Rivers in Washington State, and there is fear that this total dissolved gas (TDG) "supersaturation" will increase mortality in juvenile and adult salmonids. Modifications to the spillway and/or its tailrace at some of these dams have or will be installed to help lower the concentration of total dissolved gas downstream of the dams. At Wanapum Dam on the mid-Columbia River, spillway modifications were designed and evaluated using a combination of physical and numerical models. The physical model provided information about the hydraulics associated with different spillway modifications. The numerical model calculated the concentration of total dissolved gas based upon hydrodynamic data from the physical model and mass transport relations developed for air-water flows. This article describes the numerical model development and application. A companion article (Mannheim and Weber, 1998) describes the physical modeling efforts made to evaluate the performance of the proposed spillway modifications at the dam.