Publication Summary

This Total Maximum Daily Load (TMDL), jointly issued by Oregon and Washington, addresses total dissolved gas (TDG) in the mainstem Lower Columbia River from the Snake River to its mouth. Elevated TDG levels, which can cause "gas bubble trauma" in fish, are caused by spill events at hydroelectric projects, both within the TMDL area and above the upstream boundary. This TMDL sets allocations in terms of excess pressure above ambient, which must be met at compliance locations below each dam in the TMDL area and above McNary Dam for upstream sources. An implementation plan describes short-term and long-term compliance for both ESA and TMDL requirements.

This Total Maximum Daily Load (TMDL) addresses total dissolved gas (TDG) in the mainstem Columbia River from its confluence with the Snake River to its mouth at the Pacific Ocean. The states of Oregon and Washington have both listed multiple reaches of the Lower Columbia River on their federal Clean Water Act 303(d) lists, due to TDG levels exceeding state water quality standards. The entire reach is considered impaired for TDG. Oregon and Washington are jointly issuing this TMDL and submitting it to the U.S. Environmental Protection Agency for its approval. Elevated TDG levels are caused by spill events at four hydroelectric projects on the Lower Columbia River. Water plunging from a spill entrains TDG at high levels. High TDG can cause "gas bubble trauma" in fish, which can cause chronic or acute effects, depending on TDG levels. Spills can be caused by several conditions. "Voluntary" spills are provided to meet juvenile fish passage goals. "Involuntary" spills are caused by lack of powerhouse capacity for river flows. Involuntary spills can result from turbine maintenance or break-down, lack of power load demand, or high river flows. Elevated TDG levels also enter the TMDL area at the upstream boundary from sources outside the TMDL area.

This TMDL sets a TDG loading capacity for the Lower Columbia River in terms of excess pressure above ambient. Allocations are specified for each dam and for the upstream boundary, also expressed in terms of excess pressure. Allocations for the dams must be met at points of compliance within each dam′s tailrace at a specified distance below the spillway, corresponding to the end of the aerated zone. The upstream allocation must be met in the pool above McNary dam.

An implementation plan is provided that describes short-term compliance with 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