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

Riparian Shading

Middle Fork Willamette at Jasper, OR (Photo by A. Buckley, 7-Aug-2002)

The Willamette River flow and temperature models were used to assess the effects of restoring riparian vegetation along the Long Tom River, and along selected reaches of the upper Willamette River. These effects were modeled by changing several model inputs: the tree-top elevation, the distance from the center of the river to the vegetation, and the fraction of solar radiation intercepted by that vegetation. These three model inputs vary as a function of location and are assigned separately for the vegetation on each bank of the river. The characteristics of the riparian vegetation, translated into input files for the models, were developed during model construction. Current vegetation characteristics were derived from aerial photographs and GIS techniques by Oregon Department of Environmental Quality (ODEQ) staff, then translated into model input files using methods developed by researchers at Portland State University (PSU), ODEQ, and USGS (Annear and others, 2004a; Sullivan and Rounds, 2004).

"System potential" vegetation, or the potential near-stream land cover, is the mature vegetation that should occur at a particular location, based on the soils and geologic materials that occur there. ODEQ conducted a study of potential near-stream land cover as part of the Willamette River temperature TMDL, and the results were used to predict the height and shading characteristics of system potential vegetation along the banks of all modeled river reaches (Oregon Department of Environmental Quality, 2006). System potential vegetation was used in the modeling of Natural Thermal Potential baseline conditions in the TMDL and in this investigation. System potential shade input files for the models were used as received from ODEQ; shade files representing current conditions were obtained from PSU as used in the latest model calibration runs.

Model results showed that restoring all riparian shading along the Long Tom River could cool the Willamette River at its point of inflow by approximately 0.03^oC, which is small but potentially useful for heat-load trades with some of the point sources upstream of Albany. Shade restoration along selected 5-mile reaches of the Willamette River upstream of Albany showed cooling effects as large as 0.19^oC at certain locations. A 5-mile reach can be traversed in a few hours, however, which caused the cooling effects to exhibit a nodal pattern with downstream distance. The cooling effect was minimal at one-half day of travel time downstream of the restored reach because the water passed by that reach during the night when shading does not particularly affect the heat budget of the river. This pattern in downstream cooling effects has potentially important ramifications for heat-load trading and the siting of riparian restoration projects.

For more information and discussion of results, see the USGS report:

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/]