Publication Summary

This Total Maximum Daily Load (TMDL) addresses total dissolved gas (TDG) in the Snake River from Idaho to the Columbia River. Elevated TDG levels, which can cause "gas bubble trauma" in fish, are caused by spills at four dams. Loading capacities and load allocations are set in terms of (1) percent saturation for fish passage conditions, and (2) excess pressure above ambient during non-fish passage conditions. Allocations must be met below each dam at a specific distance below the spillway (near the end of the aerated zone). The implementation plan describes short-term and long-term compliance with both Endangered Species Act and TMDL requirements.

This Total Maximum Daily Load (TMDL) addresses total dissolved gas (TDG) in the mainstem Snake River from its confluence with the Clearwater River to its mouth at the Columbia River. Washington State has listed multiple reaches of the Lower Snake River on its federal Clean Water Act 303(d) list due to TDG levels exceeding (violating) state water quality standards. The entire reach is considered impaired for TDG. Washington State is issuing this TMDL and submitting it to the U.S. Environmental Protection Agency for its approval.

Spill events at four hydroelectric projects on the Lower Snake River – Lower Granite, Little Goose, Lower Monumental, and Ice Harbor dams – elevate TDG to levels that violate state standards. Water plunging from a spill entrains air and carries it to a depth where hydrostatic pressure forces gas into solution at high levels. High TDG can cause "gas bubble trauma" in fish, which can cause chronic or acutely lethal effects, depending on TDG levels. A spill can be caused by several conditions. A "voluntary" spill is provided to meet juvenile fish passage goals. An "involuntary" spill is caused by lack of powerhouse capacity for river flows. An involuntary spill can result from turbine maintenance or breakdown, lack of power load demand, or high river flows. The load allocations do not apply at river flows greater than the 7Q10 flow rate. Measurements of TDG levels in the pool at the upstream boundary of the TMDL area occasionally exceed standards, and appear to be related to solar heating and photosynthesis in the Lower Granite pool.

This TMDL sets a TDG loading capacity for the Lower Snake River in terms of percent saturation for fish passage spills, and in terms of excess pressure above ambient during non-fish passage conditions. Load allocations also are expressed in the same terms as loading capacity, and must be met within compliance areas above and below each dam, from the Idaho border to the Columbia River. The compliance area boundary at each dam′s tailrace begins at a specified distance below the spillway, corresponding to the end of the aerated zone. Allocations are set for each dam′s tailrace and forebay under fish passage spills, and for the entire river during non-fish passage conditions.

An implementation plan is provided that describes short-term compliance with the federal Endangered Species Act requirements. Long-term compliance is described for both Endangered Species Act and TMDL requirements.

Link to EIM data for User Study ID PPI0005