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Scientific Investigations Report 2007–5185

Scientific Investigations Report 2007–5185

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The water-temperature standard for the State of Oregon was designed to protect the needs of targeted fish species during critical periods when they use rivers for spawning, rearing, migration, or other life stages (Oregon Department of Environmental Quality, 2007a). Many rivers in western Oregon, including the Willamette River and many of its tributaries, exceed the maximum water-temperature standard, most typically during summer when salmonids are rearing or migrating, or during spring or autumn when salmonids are spawning. The Federal Clean Water Act requires that exceedances of water-quality standards be addressed, and in this case a plan of remediation was required for the Willamette River under the Act’s Total Maximum Daily Load (TMDL) provisions.

In September of 2006, after many years of data collection and modeling, the Oregon Department of Environmental Quality (ODEQ) finalized the Willamette temperature TMDL (Oregon Department of Environmental Quality, 2006a and 2006b). 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 (fig. 1). 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.

Dams have an important effect on flow and temperature in the rivers downstream of those projects (Collier and others, 1996). The U.S. Army Corp of Engineers built and operates a system of 13 dams in the Willamette River basin that provides flood control, recreation, power production, and summertime flow augmentation for navigation, among other uses. Many of these dams are tall enough, and their point of release is low enough, that the temperature of water releases in July and August typically is far cooler and the temperature of releases in September and October typically is far warmer than what would occur in the absence of the dam (Sullivan and Rounds, 2004). The direct temperature effect diminishes with distance downstream, though the effect is still measurable for many tens of miles or more, depending on various factors. The upstream dams in the Willamette River system were given monthly temperature targets under the TMDL in an attempt to restore a more natural seasonal temperature pattern and ensure compliance with the temperature standard at that point and in nearby downstream reaches.

A major nonpoint source of heat to the Willamette River and its tributaries is a degraded level of riparian shading that allows a greater amount of solar radiation to be absorbed by adjacent rivers. Under the TMDL, riparian vegetation is required to be restored to a more natural level, calculated using information on the types of vegetation that typically grow on certain surficial geologic units and accounting for some natural level of disturbance.

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), defined under the State of Oregon water-temperature standard (Oregon Department of Environmental Quality, 2007b) as

“the determination of the thermal profile of a water body using best available methods of analysis and the best available information on the site-potential riparian vegetation, stream geomorphology, stream flows, and other measures to reflect natural conditions.”

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. Water withdrawals for municipal, agricultural, and industrial uses were included in the NTP baseline conditions, as were the effects of upstream dams. A more historic channel shape was not included in the TMDL definition of NTP.

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. That assessment was done only for the most critical conditions, when the rivers exceeded the numeric criteria of the temperature standard under NTP conditions. Under such conditions, Oregon’s temperature standard specifies that NTP temperatures become the applicable temperature criteria, and anthropogenic heating effects must be limited to a small amount (0.3°C), called the Human Use Allowance (HUA). Using this type of analysis and a policy decision specifying that 0.23°C of the HUA could be assigned to the point sources, ODEQ used the models iteratively to determine 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.

Trading of heat allocations among the dischargers and designated management agencies, including direct point-source trading or point-source to nonpoint-source trading, may be an efficient means of meeting the obligations of the point sources under the Willamette temperature TMDL while also improving the river ecosystem. A framework for creating a marketplace for trading “ecosystem service” credits, including temperature credits, is being pursued by the Willamette Partnership ( Quantitative tools are needed, however, to assess the temperature effects of any proposed action or trade. 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.

Purpose and Scope

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:

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. This report documents the results of this investigation.

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 (fig. 1); 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.

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