Willamette River Basin Temperature TMDL Modeling Study

The Corps of Engineers dynamic 2-D model CE-QUAL-W2 Version 3.1 was applied to the Willamette River basin in Oregon. The Corps of Engineers and the State of Oregon Department of Environmental Quality (DEQ) funded the modeling and TMDL study. The study area included about 1000 km (600 miles) of river. The model domain included the mainstem Willamette River, the North and South Santiam Rivers, the Long Tom River, the McKenzie River and the Coast and Middle Forks of the Willamette River. The model domain also included about 70 miles of the Columbia River since it affects the tidally influenced portion of the lower Willamette basin. This TMDL project focused on meeting temperature standards and will form the basis for a water quality study at a later time.The study included model construction (grid development, shading data analysis, meteorological data, and dynamic inflow boundary conditions of flow and temperature), model calibration (model-data comparisons of flow rate and water level at monitoring stations, dye studies performed throughout the basin, and continuous temperature data at dozens of monitoring locations), and evaluation of modeling strategies for temperature improvement. These management scenarios included evaluating the impacts of stream shading, different flow management practices from storage reservoirs in the headwaters of the Willamette, and the impact of point source discharges (primarily wastewater treatment plants, pulp and paper mills, and various industries).The model development and evaluation of alternatives was also reviewed by the Willamette River TMDL Council, an Oregon citizen and stakeholder group evaluating the Willamette River water quality. The USGS, DEQ, and Portland State University modeling team worked together to provide peer review and model refinement, although DEQ ultimately had responsibility to establish the TMDL assessment approach.