Spill Caps

Spill Cap Objective: When spilling water at dams is necessary to aid fish passage, Total Dissolved Gas, TDG, must not exceed an average of 115% as measured at Camas/Washougal below Bonneville dam or as measured in the forebays of the next downstream dams. TDG must also not exceed an average of 120% as measured in the tailraces of each dam. These averages are based on twelve highest hourly readings in any one day of TDG. In addition, there is a maximum total dissolved gas one hour average of 125%, relative to atmospheric pressure, during spillage for fish passage.

The Water Quality Unit of the Corps Reservoir Control Center is responsible for monitoring the TDG in the forebays and tailwaters of the Lower Columbia and Snake River dams, and selected river sties. To avoid TDG exceedances, the Spill Caps are changed accordingly after a review of the following: project data at the Dams, project powerhouse and spillway characteristics (where applicable), current and near-future special operations, current TDG levels in the forebays and tailwaters, water temperatures, short and long-term weather forecasts, and the resulting model simulations.