Temperature Effects of Point Sources, Riparian Shading, and Dam Operations on the Willamette River

Cougar Dam Operations

Cougar Dam: aerial view (Photo: US Army Corps of Engineers, 12-Jul-1989)

The downstream temperature effects of changed operations at Cougar Dam on the South Fork McKenzie River were evaluated using the TMDL models. Since 2005, temperature releases from Cougar Dam have been controlled through the use of a selective withdrawal tower; this change occurred after the 2001-2002 time period modeled for the temperature TMDL. The downstream thermal effects were estimated by substituting 2006 measured temperatures downstream of Cougar Dam for the measured 2001 and 2002 temperatures used in the TMDL models. The model results showed that the modified operations at Cougar Dam have an important effect on downstream temperatures, with warming in mid-summer and cooling in autumn. Predicted 7dADM (7-day moving average of the daily maximum) temperature shifts in the South Fork McKenzie River were greatest, with changes as large as 6.0^oC. Downstream, the models showed 7dADM temperature changes in the McKenzie River of close to 2.0^oC at times. In the Willamette River upstream of the Santiam River confluence, predicted 7dADM temperature changes were as large as 0.5^oC, while downstream of the Santiam River the changes were less than 0.3^oC. Although large temperature shifts were apparent as a result of the change in operations at Cougar Dam, if those changes also were incorporated into the Natural Thermal Potential baseline conditions, the modeled cumulative point-source heating effect was not significantly altered.

For more information and a detailed discussion of results, see the following USGS reports:

Rounds, S.A., 2007, Temperature effects of point sources, riparian shading, and dam operations on the Willamette River, Oregon: U.S. Geological Survey Scientific Investigations Report 2007-5185, 34 p.
[Available online at: http://pubs.usgs.gov/sir/2007/5185/]

Rounds, S.A., 2010, Thermal effects of dams in the Willamette River basin, Oregon: U.S. Geological Survey Scientific Investigations Report 2010-5153, 64 p.
[Available online at: http://pubs.usgs.gov/sir/2010/5153/]