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Scientific Investigations Report 2007–5185

Scientific Investigations Report 2007–5185

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Temperature Effects of Dam Operations

In addition to assessing the effects of point-source heat discharges and riparian shading, the Willamette River flow and temperature models were used to assess the thermal effects of changed operations at Cougar Dam, which is one of the upper boundaries for the McKenzie River model. Situated on the South Fork McKenzie River and completed in 1963, Cougar Dam is the second highest dam (452 ft) and impounds the fifth largest reservoir (219,000 acre-ft) in the Willamette River basin (fig. 10).

Cougar Dam controls the flow and greatly influences the temperature in the South Fork McKenzie River downstream of the dam. Cougar Reservoir becomes thermally stratified in summer, with warmer, less-dense water near the surface and colder, more-dense water at the bottom (Resource Management Associates, 2003). Western Oregon’s warm and sunny summer weather adds additional heat to the reservoir’s surface, stabilizing its stratification throughout the summer. Because the dam was built with its major release point at a relatively low elevation, the dam historically released relatively cold water from near the bottom of the reservoir in mid-summer. As the reservoir was drawn down in autumn to make room for flood-control storage, the heat that was captured in the reservoir’s upper layer during the summer was released downstream. As a result, the seasonal temperature pattern downstream of Cougar Dam through 2001 was quite different from the pattern upstream of Cougar Reservoir (fig. 11.).

The McKenzie River supports the largest remaining wild population of Chinook salmon (Oncorhynchus tshawytscha) in the upper Willamette River basin (Good and others, 2005), and the South Fork McKenzie River provides good spawning habitat. The altered temperature pattern downstream of Cougar Dam, however, can create problems with regard to the timing of migration, spawning, and egg hatching (Caissie, 2006). To restore the suitability of this reach for salmonid spawning, the U.S. Army Corps of Engineers (USACE) added a sliding gate assembly to the intake structure at Cougar Dam. To allow for construction, the reservoir was drawn down from 2002 through 2004; construction was completed in early 2005. The new selective withdrawal tower allows dam operators to blend warm water from the top of the reservoir with cooler water at other levels, or to simply select a depth from which to withdraw water, in an attempt to match a downstream temperature target. The selective withdrawal tower was used successfully in 2005 and 2006 to restore a more-natural seasonal temperature pattern to the South Fork McKenzie River downstream of Cougar dam (fig. 12).

The time periods modeled as the basis for the Willamette temperature TMDL included one summer in which Cougar Dam operated without a selective withdrawal tower (2001), and one summer during which Cougar Reservoir was drawn down for construction (2002). Release temperatures in 2001 were typical of the pre-selective-withdrawal-tower period, with cool releases in mid-summer and warmer releases in autumn. In 2002, the greatly reduced storage and shallower depth in Cougar Reservoir resulted in a short residence time and less stratification that might affect release temperatures. As a result, 2002 release temperatures from Cougar Dam mirrored the seasonal temperature pattern upstream, with limited warming in the smaller reservoir. These measured temperatures were used as upstream boundary conditions for the South Fork portion of the McKenzie River model used for the Willamette temperature TMDL. Cougar Dam’s current and future release temperatures, however, are unlikely to resemble those that occurred in 2001; therefore, it is instructive to explore how these seasonal changes in Cougar Dam release temperatures affect downstream temperatures in the McKenzie and Willamette Rivers, and whether such changes might affect future point-source heat allocations under the Willamette temperature TMDL.

To determine the effect of changed operations at Cougar Dam on downstream water temperatures, the Willamette flow and temperature models were used to answer one simple question: “What might the temperatures downstream of Cougar Dam have been during 2001 and 2002 if the released temperatures during those periods had been equal to those measured in 2006?” Release temperatures in 2005 and 2006, the first full years after completion of the selective withdrawal tower, were similar; the 2006 water temperatures were selected because the second year had fewer operational glitches and should be more representative of future seasonal patterns. Obviously, superimposing 2006 release temperatures upon flow conditions from 2001 and 2002 is imperfect. Release temperatures are tied somewhat to the flows, as a stratified reservoir has a limited supply of warm and cool water. In addition, the 2006 release temperatures are tied somewhat to the meteorological conditions that occurred during 2006. Still, running the 2001 and 2002 models with 2006 release temperatures is a good first step toward quantifying the thermal effects of changed operations at Cougar Dam. Further refinement of these results can be an objective for future research.

Imposing 2006 release temperatures on the 2001 modeled conditions has a greater effect than when imposed on the 2002 conditions, for two main reasons. First, differences between 2001 and 2006 water temperatures are greater than differences between 2002 and 2006 water temperatures (fig. 12). Second, the amount of water discharged from Cougar Reservoir was much less than normal during August–October 2002 because the reservoir was drawn down for construction. Late summer releases in 2002 were less than 250 ft3/s, while releases during that time period in 2001 exceeded 700 ft3/s. The greater flows in 2001 have a greater effect on downstream temperatures once those flows are mixed into the McKenzie and Willamette Rivers.

The Willamette River flow and temperature models first were used to determine the change in 7dADM water temperature downstream of Cougar Dam in the absence of point sources. All other conditions, except for the modified temperatures released from Cougar Dam, were identical to those used in the Willamette TMDL. Results from those model runs showed that Cougar Dam’s selective withdrawal tower has the greatest effect on water temperatures in the South Fork McKenzie River, with mid-summer 7dADM temperature increases as large as 6.0–6.5°C and decreases of 5.0°C or more in October, as compared to the original 2001 model results (fig. 13). As expected, the modeled temperature changes for the 2002 model runs were smaller than those for 2001. Downstream of the confluence of the South Fork McKenzie River with the McKenzie River, the thermal effects are somewhat diluted, though the 7dADM temperature changes in the McKenzie River were still large enough to be important, relative to the temperature modifications mandated by the TMDL. McKenzie River 7dADM temperatures were warmed in mid-summer as much as 1.5 or 2.0°C, depending on time of year and location, and cooling in autumn was sometimes more than 1.5°C. Because of additional dilution and time for heat exchange with the atmosphere, the magnitude of the temperature effect decreased downstream of the confluence of the McKenzie River with the Willamette River (fig. 14). In the Willamette River, 7dADM water-temperature changes as large as 0.4–0.5°C (warming in summer, cooling in autumn) were predicted upstream of the Santiam River confluence (RM 108.5). The effect diminished to no more than 0.3°C downstream of that point.

In addition to imposing 2006 release temperatures, the models were run both with and without the point sources to determine their cumulative heating effect under the modified baseline conditions. As in the TMDL analysis, the cumulative heating effect was determined by first calculating the 7-day mean of the modeled daily maximum temperature for each location in the models and for each day that was simulated. Then, the difference in the 7dADM temperature for each day and location was calculated by subtracting the without-point-sources results from the with-point-sources results. Finally, the 95th percentile of the 7dADM temperature-difference data at each location was calculated and plotted against downstream distance along the McKenzie and Willamette Rivers. These results for both the original TMDL point-source model runs (“TMDL base case”) and for the model runs where the 2006 Cougar Dam release temperatures were imposed (“Cougar retrofit”) are shown in figure 15.

Although the changed operations at Cougar Dam were shown to have a significant effect on downstream temperature in the McKenzie River system and potentially measurable effects in the Willamette River, incorporating the Cougar Dam retrofit into the cumulative point-source temperature assessment had little effect. The difference between the “base case” and “Cougar retrofit” point-source temperature effects was determined to be small, ranging from -0.006 to 0.008°C, with less than a 0.001°C decrease at the POMI near RM 117 (fig. 15). The effect is small because the Cougar Dam retrofit was incorporated into both the with- and without-point-sources model runs. The temperature of the river receiving the point-source flows was slightly different as a result of the Cougar Dam retrofit, but not so different that the cumulative point-source effects were greatly affected. Because the cumulative point-source heating effects were not greatly affected by the Cougar Dam retrofit, the point-source heat allocations of the Willamette temperature TMDL would not likely change appreciably if the Cougar Dam retrofit had been included in the TMDL analysis. Only for those point sources closest to Cougar Dam, and particularly for those located on the McKenzie River, might a more detailed analysis be warranted.

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