The Project Effect on TDG. The Columbia River did not always meet the numeric criteria for TDG before the Project existed, and would not always meet them even if the Project was removed. This is due to the modifications to the river caused by the construction of upstream hydroelectric and storage projects, which result in elevated TDG levels before the water enters the Project boundary. The Project has an effect on TDG when spilling water, but the effect differs depending on the circumstances. Years of monitoring show that when TDG levels in water reaching the Project are near or below 110%, the Project’s spill operations typically increase the TDG level at the downstream fixed monitoring site (DFMS), located four miles downstream of the Project, by 1- 3%. When TDG levels arriving at the Project are between 115-120%, spill operations at the Project generally do not affect the average TDG levels at the downstream Rock Island Project. When TDG levels arriving at the Project exceed 120%, the Project’s spill operations typically reduce TDG levels arriving at the Rock Island Dam. Regression analysis indicates that the Project can meet TDG numeric criteria at all flows up to the 7Q10 flow. The Project has reported exceedances of the TDG criteria since 1997 (Table 2-1). Over that time period, exceedances were infrequent except during high flow events in 1997 and 2002, when flows frequently exceeded 200,000 cubic feet per second (cfs). However, only a small number of those exceedances were caused by the Rocky Reach Project. This record demonstrates that during high flow periods when upstream dams raise TDG levels above 120%, presenting the most potential risk to fish, the Project either has no effect on TDG or reduces TDG levels in the Columbia River downstream from the Project. The record of TDG exceedances demonstrates that the level of spill used by the Project to increase downstream fish passage survival has been successfully managed to meet water quality numeric criteria. From 1997 to 2004, there have been 140 exceedances of TDG in water arriving at the Project’s forebay. Although Rocky Reach was also spilling when these exceedances occurred, the number of exceedances was lower at the Rocky Reach Project’s DFMS (102) and at the Rock Island Project’s forebay (137). An analysis of the exceedances below the Project during these years found that only 11 of the 102 exceedances of the 120% criterion at the DFMS and 17 of the 137 exceedances of the 115% Rock Island forebay criterion were caus...
