Publication Summary

This Total Maximum Daily Load (TMDL), developed jointly by Washington State, the Spokane Tribe of Indians, and EPA, addresses total dissolved gas (TDG) in the Columbia River and Lake Roosevelt from the Canadian border to the Snake River. Elevated TDG levels, which can cause "gas bubble trauma" in fish, are caused by spills from Mid-Columbia dams and by upstream sources. Separate allocations apply to fish passage and non-fish passage conditions. Allocations must be met below the spillway of each dam (near the end of the aerated zone). The implementation plan describes compliance with both Endangered Species Act and TMDL requirements.

This Total Maximum Daily Load (TMDL) study addresses total dissolved gas (TDG) in the mainstem Columbia River from the Canadian border to the Snake River confluence. Washington State has listed this area on its federal Clean Water Act 303(d) list due to TDG levels exceeding state water quality standards.

EPA is issuing this TMDL for all waters above Grand Coulee Dam and for all tribal waters. Washington State is issuing this TMDL for state waters below Grand Coulee Dam and submitting it to EPA for approval.

Elevated TDG levels are caused by spill events at seven dams on the Mid-Columbia River and by other sources upstream of the international border and in the Spokane River. Water plunging from a spill generates TDG at high levels, which can cause "gas bubble trauma" in fish. Voluntary spills are provided to meet juvenile fish passage goals. Involuntary spills are caused by lack of powerhouse capacity for river flows.

This TMDL sets TDG loading capacities and allocations for the Mid-Columbia River and Lake Roosevelt, both in terms of percent saturation for fish passage and excess pressure above ambient for non-fish passage. Allocations are specified for each dam and for upstream boundaries. Fish passage allocations must be met at fixed monitoring stations. Non-fish passage allocations must be met in all locations, except for an area below each dam (other than Grand Coulee) from the spillway downstream to the end of the aerated zone. Attainment of allocations will be assessed at monitoring sites in each dam′s forebay and tailrace and at the upstream boundaries.

A Summary Implementation Strategy prepared by the Washington State Department of Ecology and the Spokane Tribe describes proposed measures that could be used to reduce TDG levels in the Columbia River. Short-term actions primarily focus on meeting Endangered Species Act requirements, while long-term goals address both Endangered Species Act and TMDL requirements.