When water is released through the spillways of dams, air is entrained in the water, increasing the downstream concentration of total dissolved gas. Excess dissolved-gas concentrations can have adverse effects on freshwater aquatic life. The U.S. Geological Survey (USGS), in cooperation with the U.S. Army Corps of Engineers (USACE), collected total-dissolved-gas (TDG) and water-temperature data at eight sites on the lower Columbia River in 2004. Significant findings from the data include:

• Variances to the Oregon and Washington water-quality standards for total dissolved gas were exceeded on a few days at three of the monitoring sites: Camas, The Dalles forebay, and Bonneville forebay. These exceedances may have been the result of the cumulative effects of supersaturated water moving downstream through the lower Columbia River. Apparently, the levels of TDG did not dissipate rapidly enough downstream from the dams before reaching the next site.
• TDG levels at an experimental monitoring site directly below Bonneville Dam at Cascade Island showed a larger response to spill than the site 5.5 miles farther downstream at Warrendale.
• From mid-July to mid-September, water temperatures were above 20°C (degrees Celsius) at each of the seven lower Columbia River sites. Both the Oregon and Washington water-quality standards contain a numerical standard of 20°C for the lower Columbia River.
• The new location of the forebay monitoring site at John Day navigation lock showed less daily temperature variation than the previous location. The probe at the new site was farther away from the dam and at a greater depth, so it apparently avoided the daily temperature excursions associated with the surface-layer heating at the previous site.
• Most field checks of total-dissolved-gas sensors with a secondary standard were within ±1% saturation. Most of the field checks of barometric pressure were within ±1 mm Hg (millimeter of mercury) of a secondary standard, and water temperature field checks were all within ±0.1°C.
• For the seven monitoring sites used to regulate spill in water year 2004, an average of 99.0% of the total- dissolved-gas data were received in real time by the USGS satellite downlink and were within 1% saturation of the expected value, based on calibration data, replicate quality-control measurements in the river, and comparison to ambient river conditions at adjacent sites.