As part of the TMDL process scientists studied the Columbia and Snake system to understand what causes increased temperatures. They analyzed everything from global warming to paper mills.

They found that the biggest influence on temperature is hydroelectric reservoirs. Scientists estimate that the reservoir (Lake Roosevelt) behind Grand Coulee can increase water temperature in the Columbia River by as much as 6 degrees Celsius above natural conditions, the most of any of the facilities. Hydroelectric facilities are grouped into the category of non-point source pollution. Other examples of non-point source pollution are erosion and removal of shading vegetation next to waterways.

Another source of pollution is called point source. These are discharges directly into the water, like waste water treatment plants and paper mills. There are 202 point source discharges in this part of the Columbia, but as the graphic shows, the effect of all these point sources combined is minor compared to the effect of reservoirs behind hydroelectric projects.