Publication Summary

Ecology monitored total dissolved gas (TDG) in the Spokane River during 2003, and compared these and other data to water quality standards. Natural processes generate TDG at Post Falls Dam. Upriver Dam generates TDG levels either below the 110% criterion or below upstream conditions. Long Lake Dam generates elevated TDG but Little Falls Dam contributes no additional TDG. Instead of a TMDL, FERC relicensing 401 certifications are adequate to address impairments. Spokane River TDG should be moved from Category 5 (TMDL required) to either Category 2 (waters of concern) or Category 4B (has a pollutant control plan).

Total dissolved gas (TDG) monitoring in the Spokane River from 1999-2003 was evaluated to determine whether Upriver Dam and Little Falls Dam are contributing to elevated TDG levels that exceed Washington State water quality criteria.

TDG is generated by the natural waterfalls at Post Falls, Idaho, and the bypass of flows through Avista′s Post Falls power plant has likely reduced TDG levels. The pool behind Upriver Dam reduces dissipation of TDG produced by Post Falls, but not enough for TDG to exceed natural levels. Upriver Dam does not appear to be increasing TDG above the 110% criterion nor is it increasing TDG above natural conditions when TDG levels are above 110%.

At Little Falls, elevated TDG can be entirely attributed to Long Lake Dam operations; no evidence can be found that Little Falls contributes additional TDG.

Based on this analysis, a TDG Total Maximum Daily Load (TMDL) for the Spokane River is not necessary. The 401 certification process for Federal Energy Regulatory Commission (FERC) hydropower relicensing should serve as a pollutant control plan sufficient to address TDG impairments in the Spokane River. Future Washington State 303(d) listings for TDG in the Spokane River should be moved from Category 5 (TMDL required) to: