U.S. Geological Survey

Effects of Hypothetical Management Scenarios on Simulated Water Temperatures in the Tualatin River, Oregon, 1998

By John C. Risley

USGS Water-Resources Investigations Report 00-4071, 110 pages, 74 figures, 22 tables

Report availability

SIGNIFICANT FINDINGS

Water temperature is one of the most important factors determining the health of fish and other aquatic organisms. If water temperatures warm beyond a critical threshold, particularly during the sensitive life stages of fish, survival can markedly decrease. In 1996, the State of Oregon adopted a revised maximum water temperature standard of 17.8^oC (degrees Celsius) (64^oF [degrees Fahrenheit]) for most waterways, including the Tualatin River in northwestern Oregon. To assess water temperature conditions in the Tualatin River, a recent cooperative study between the U.S. Geological Survey and the Unified Sewerage Agency of Washington County, Oregon, used two dynamic-flow heat-transport models, DAFLOW-BLTM (river mile [RM] 63.9-RM 38.4) and CE-QUAL-W2 (RM 3 8.4-RM 3.4). After the models were calibrated with data collected during the 1994 low-flow season, they were used to simulate various hypothetical water-management scenarios. Results from the first 10 scenarios were published in an earlier report. This report presents the results of an additional 16 scenarios for both 1994 and 1995 conditions. In all 16 scenarios, the State's temperature standard (17.8^oC) was exceeded in much of the lower reaches of the Tualatin River during the warmer months in both years.

• The effect of diverting 1.33 ft³/s (cubic feet per second) of Rock Creek Wastewater-Treatment plant (WWTP) effluent for irrigation was evaluated. Temperatures downstream of that facility (RM 38.1) for most months decreased about 0.05^oC or less. Farther downstream, near RM 10, the effect was almost negligible. The effect of the diversion is slightly more apparent in the 1994 simulation than in the 1995 simulation. In a similar follow-up scenario, a constant flow of 1.33 ft³/s was withdrawn from the river at RM 37.3 and an additional constant flow of 2.0 ft³/s was released from Henry Hagg Lake to compensate. The effect of this diversion/augmentation on the river system was also fairly minimal for both 1994 and 1995. Temperatures generally decreased from RM 60.0 to RM 3.4 by about 0.05 to 0.1^oC. For most months, the overall cooling resulting from this scenario was slightly greater than the cooling resulting from the former scenario.
• In another set of scenarios, the effect of piping and then releasing Rock Creek WWTP effluent at two upstream locations (RM 43.8 and RM 55.2) was evaluated. A constant flow of 5 Mgal/d (million gallons per day) was released at each upstream location, in addition to a constant release of either 10, 20, or 30 Mgal/d of effluent at RM 38.1. Temperatures increased between RM 55.2 and RM 38.1 by about 1.0^oC or less, but were still within compliance with the water-quality standard. Downstream of RM 38.1 the river temperature decreased (generally 0.6^oC or less) if the release from Rock Creek WWTP was only 10 Mgal/d. If the release from Rock Creek WWTP was 20 or 30 Mgal/d, temperatures downstream of RM 38.1 generally increased. However, the magnitude of the increase was generally less than 1.0^oC.
• The temperature effect resulting from constant 25, 45, or 65 Mgal/d effluent releases from the Rock Creek (RM 38.1) and Durham (RM 9.3) WWTPs was evaluated. Temperatures throughout the reach downstream of Rock Creek WWTP and, to a lesser extent downstream of Durham WWTP, increased proportionately. The magnitude of the increases was as much as 0.6, 1.5, and 2.2^oC for the three scenarios, respectively.
• In another scenario, a cooler water-temperature data set, representing more shaded "natural" background conditions, was used as input to the model upper boundary at Gaston (RM 63.9). Water temperatures decreased substantially between RM 63.9 and the confluence with Scoggins Creek (RM 60.0) by as much as 4.0^ooC. In a follow-up scenario, the same model upper boundary condition was used in conjunction with the "natural" background conditions scenario from an earlier study. Water temperatures again decreased substantially between RM 63.9 and the confluence with Scoggins Creek (RM 60.0). However, between Scoggins Creek and the Dairy Creek confluence (RM 44.8), water temperatures gradually increased because the unnaturally cool water released from Henry Hagg Lake was not present. However, almost all of the reach above Rood Bridge (RM 38.4) was still in compliance with the water-quality standard. Below RM 38.4 temperatures increased (1.0^oC or less) for July and August and decreased for other months.
• The effect of setting the temperature of effluent released at RM 38.1 and RM 9.3 equal to the temperature of the river was evaluated. Temperatures downstream of RM 38.1 decreased by as much as 2.4^oC. The reduction then tapered off to 0.5^oC upstream of RM 9.3. Downstream of RM 9.3, temperatures decreased by as much as 1.2^oC.
• Another scenario was used to evaluate the effect of releasing a purchased allotment of Scoggins Dam flow (up to, but not exceeding 10 Mgal/d) at RM 38.1 instead of into Scoggins Creek. Observed Scoggins Dam temperature data were used for the allotted flow. Temperatures increased for all months except October from RM 60.0 to RM 38.1 by as much as 0.6^oC. However, downstream of RM 38.1, temperatures decreased from as much as 0.7^oC for all months except October. However, the effect of the supplemental release became less pronounced farther downstream.
• The effect of constant effluent releases of 20, 25, 45, and 65 Mgal/d at two WWTPs (RM 38.1 and RM 9.3) was evaluated. The 1994 and 1995 measured effluent temperature data from the WWTPs were used, except that the temperatures were not permitted to be greater than 17.8^oC. For most months, the temperature in the reach downstream of both WWTPs decreased in all four scenarios. From RM 38.1 to RM 9.3, the temperature decrease was less than 1.0^oC. Downstream of the Durham WWTP (RM 9.3), temperatures decreased almost by 2.0^oC.

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