Dissipation of Supersaturated Total Dissolved Gases in the Intermediate Mixing Zone of a Regulated River

In regulated rivers downstream of hydropower facilities, understanding how supersaturated total dissolved gases (TDGs) are distributed and dissipated is crucial, as they can negatively impact the aquatic environment. The objective of this study was to quantify the rate of dissipation of supersaturated TDG in the regulated Columbia and Kootenay Rivers and to evaluate whether it can be predicted by surface reaeration theories. Detailed measurements of TDG and river hydraulics were collected during two individual spill events conducted at the Hugh L. Keenleyside Dam (HLK) on the Columbia River and the Brilliant Dam on the Kootenay River. To estimate the dissipation rate, an analytical approach was used that incorporated transverse mixing between the spill and generation flow as well as tributary inflow. For four different gate operations at the HLK Dam, the average rate in the two hydraulically different reaches upstream and downstream of Kootenay River confluence was 0.004 and $0.021{\mathrm{h}}^{-1}$, respectively. In the downstream reach, the rate was $0.038{\mathrm{h}}^{-1}$ during the spill operation at the Brilliant Dam. These rates were about 1.5–3 times higher than the surface transfer predicted by some well known reaeration models. Some potential causes of these higher rates were discussed, particularly the large variation in the downstream reach. Because of limited data availability, a conceptual argument based on gas transfer theories was presented, which indicated that the dissipation rates were likely enhanced by bubble-mediated transfer caused by liquid phase supersaturation.