TDG supersaturation often happens in the spring when melting snow pack creates heavy flows and/or flooding. Water that can not be stored in the reservoir behind a dam or passed through turbines to generate electricity is sent over the spillway. TDG supersaturation occurs as the water plunges into the basin below.

Hydroelectric operators also spill water to help downstream salmon migrate to the ocean. And sometimes spill occurs because maintenance within a powerhouse forces operators to send water over a spillway. So while TDG supersaturation is most common in the spring time, it can happen during other seasons as well.

Along the Columbia, including Lake Roosevelt, an extreme case of TDG supersaturation occurred in 1997. This was a year when we had lots of snow pack that melted quickly in late April. In both Canada and the United States, this forced dam operators to open their spill gates and allow the water to come rushing through. TDG supersaturation soared, killing thousands of fish.