Summertime stream temperatures in the North and South Forks of the Sprague River, south central Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mp48sh466

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  • The Upper Sprague River Watershed (North and South Forks of the Sprague River) in south central Oregon provides important habitat for salmonid species, including native bull trout (Salvelinus confluentus) and redband trout (Oncorhynchus mykiss ssp.). Concern over the loss of viable habitat for these species has increased due to reductions in channel and habitat complexity, and modification of riparian vegetation. Many of the factors affecting habitat quality also influence water temperature, which is an important habitat component of salmonids during the summer months. Maximum stream temperatures and diel fluctuations in the Upper Sprague River System generally reflected local reach characteristics, position in the drainage, and large scale changes in valley shape. Stream temperatures in the North Fork Sprague River (NFSR), at a distance of 35 km from the drainage divide, equalled or exceeded 15°C, the 'upper preferred temperature' for salmonids, 16% and 36% of the time during the seven-consecutive warmest days of 1993 and 1994, respectively. Upstream of Lee Thomas Meadow (LTM), 6.5 km from the drainage divide, 15°C was equalled or exceeded only 1% and 13% of the time during the same period, while below LTM (18.3 km from the drainage divide), 15°C was exceeded 66% and 85% of the time during this sante period. Diel fluctuations in the NFSR were greatest immediately below LTM (>11.8°C), decreasing to <6.0°C below the canyon section at the mouth of the watershed during both 1993 and 1994. In the South Fork Sprague River (SFSR), 15°C was equalled or exceeded 79% and 99% of the time during the seven-consecutive warmest days of 1993 and 1994, respectively, 28.4 km from the divide. At this same distance, the upper lethal limit of 25°C was equalled or exceeded 16% of the time in 1994, representing very stressful or potentially life-threatening conditions for salmonids. Diel fluctuations for the SFSR were >6.5°C both years, the highest values being 9.5°C or greater at 28.4 kin from the divide. Seven-day maximum stream temperatures and diel fluctuations in tributaries of the Upper Sprague River System varied widely during the study period. Maximum stream temperatures varied from 13.8°C to 23.1°C while diel fluctuations varied from 4.8°C to 11.0°C; the highest values corresponding with relatively open, unshaded reaches and the lower values corresponding with shaded, forested reaches. Relationships between stream temperatures, stream cover and channel morphology characteristics were evaluated for eight reaches within the study area. Only stream cover was found to be significantly (p <0.05) related to seven-day maximum stream temperatures, change in stream temperatures, and diurnal fluctuations in stream temperatures based on simple linear regression. When multiple linear regression analysis was used, several combinations of independent variables were found to be significantly (p <0.05) related to seven-day maximum stream temperatures, and/or change in stream temperatures, and/or diurnal fluctuations in stream temperatures. The variables that were consistently part of significant (p <0.05) regressions included: stream cover, wetted width, thalweg depth, width/depth ratio, and reach length. A stream temperature model, SHADOW, was found to be a relatively good predictor of maximum stream temperatures for short (1130 m) reaches and generally a poor predictor for longer (1340-4130 m) reaches. For the three short reaches (1130 m), the average difference between observed and modelled temperatures was 0°C (±0.3°C); for the five longer reaches (1340-4130 m), the average difference was 2.9°C (±0.6°C) indicating that the SHADOW Model over-predicted stream temperatures for these longer reaches. The shortest forested reach was under-predicted by 0.4°C. Temperature simulations for five meadow reaches using SHADOW and multiple regression models suggest that lower maximum stream temperatures would be observed if stream cover were increased.
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