Temperature Effects of Point Sources, Riparian Shading, and Dam Operations on the Willamette River
In September of 2006, after years of data collection and modeling, the Oregon Department of Environmental Quality (ODEQ) finalized the Willamette temperature TMDL. A large part of the TMDL focuses on the main-stem Willamette River and selected major tributaries (Fall Creek as well as the Clackamas, Santiam, North Santiam, South Santiam, Long Tom, McKenzie, South Fork McKenzie, Coast and Middle Fork Willamette, and Row Rivers) as far upstream as the first major dam on each tributary (map). The TMDL is meant to regulate several important sources of temperature alteration in this system, including upstream and instream dams, riparian vegetation, and point-source discharges.
The thermal effects of both point-source discharges and riparian shading were assessed for the TMDL by using a set of flow and temperature models developed for that application. The effects of the point sources were evaluated relative to a baseline condition termed Natural Thermal Potential (NTP, Oregon Department of Environmental Quality, 2007b). Essentially, NTP represents the water temperature that would occur in a stream if certain anthropogenic influences were either minimized or eliminated. For the Willamette temperature TMDL, NTP conditions were defined as the water temperatures that would occur in the absence of point sources, with restored riparian vegetation, without Portland General Electric's cap and flashboards at Willamette Falls, and without the Eugene Water and Electric Board's hydroelectric diversions on the McKenzie River.
Using the Willamette flow and temperature models, NTP conditions were defined for a modeled time period in 2001 and 2002, and the cumulative thermal effects of the largest point sources were assessed. ODEQ then used the models iteratively to determine a set of maximum heat-load allocations for each of the permitted point-source facilities.
In the final TMDL, many of the point sources' heat allocations are sufficiently restrictive that accommodating current conditions and future growth may be difficult without corrective action or an increased heat-load allocation. Several different strategies are being proposed in an attempt to accommodate existing and future heat loads. One alternative is for each point source to find ways to reduce their heat load, possibly by decreasing the amount of water discharged. For example, many municipalities have programs in which treated wastewater is piped to nearby golf courses for use as irrigation water. If the heat load contributed by a point source could be decreased, then that point source might no longer need all of its heat allocation under the TMDL. By accepting a lower allocation, a "credit" could be created that might be traded or sold to another point source that needs a higher allocation. This sort of trading is allowed under the Willamette temperature TMDL.
Quantitative tools are needed to assess the temperature effects of proposed actions or potential heat-allocation trades. In this investigation, the U.S. Geological Survey (USGS) worked in cooperation with the Oregon Association of Clean Water Agencies (ACWA) and the Willamette Partnership to address some of these temperature-related issues under the TMDL.