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

Objectives and Approach

McKenzie River abv Hayden Bridge (Photo by D. Cushman, 12-Sep-2005)

The purpose of this investigation was to develop a better understanding of the effects of point and nonpoint sources of heat as well as upstream dam operations on water temperature in the Willamette River and the lower reaches of its largest tributaries. The investigation was geared primarily toward quantifying these effects in the context of the thermal allocations set by the Willamette temperature TMDL. Specifically, the objectives of this investigation were to:

• Evaluate the efficacy of various point-source heat allocation trading scenarios, including the partial or complete removal of selected point-source discharges from the river, with the goal of allowing for future growth and the efficient use of existing capacity by other point sources;
   
• Evaluate the effect of increased point-source heat allocations on the temperature regime of the Willamette River;
   
• Evaluate the effect of selected riparian shade-restoration projects on the thermal characteristics of the Willamette River system; and
   
• Evaluate the effect of changed operations at Cougar Dam on downstream water temperatures, including the potential effect of those operations on downstream point-source heat allocations.

Through these objectives, this investigation was designed to develop a better overall understanding of anthropogenic influences on water temperature in the main-stem Willamette River, and provide information and tools that might be used in the development of a heat-trading system that operates within the limits set by the Willamette temperature TMDL.

Both the spatial and temporal scopes of this investigation were aligned with those used in the development of the TMDL. The Willamette flow and temperature models, developed previously to form the basis for the temperature TMDL, were used to simulate the time periods from June 1 through October 31 of 2001 and from April 1 through October 31 of 2002. The models include the entire main-stem Willamette River as well as the lower reaches of selected tributaries (Clackamas, Santiam, North Santiam, South Santiam, Long Tom, McKenzie, South Fork McKenzie, Middle and Coast Fork Willamette, and Row Rivers, as well as Fall Creek) up to their first major dams (map); the McKenzie River was modeled only up to the point where it is joined by the South Fork McKenzie River. By keeping the same spatial and temporal domains as those used in the Willamette temperature TMDL, the results of this investigation can be directly compared to and augment the information and results created during the TMDL process.