Spillway discharge at the spillways discharging as a free jet has been shown to produce high Total Dissolved Gas (TDG) concentrations in the tailwater of the spillway and the river reach downstream. High TDG concentrations can cause gas bubble disease in fish, and therefore is a negative environmental impact to be avoided. One challenge is that there are many unknown factors for a large free jet discharging into a pool, such as air entrainment, bubble size, rate of bubble-water transfer, and free surface transfer. Our approach has been to assume that the air entrained into the pool by the spillway and tunnel is sufficient to develop a steady state concentration of TDG, such that additional air entrainment would not alter this concentration. Thus, bubble-water desorption near the surface is balanced by absorption at deeper depths. The location of entrained air bubbles has been simulated with a computational fluid dynamics code. Gas transfer across the bubbles was then simulated with source/sink algorithms developed by the authors and applied to the bubbles. This paper will describe two predictions of TDG concentration and comparisons with field measurements.