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The Beaver Creek confluence with the main-stem Klamath River was studied to assess salmonid use in a thermal mixing zone under various summer hydrological and meteorological conditions. Main-stem flow releases from Iron Gate Dam ranged from 17 cms (615 cfs) to 37 cms (1320 cfs) during the study period and main-stem water temperatures ranged from 19.5 to 268C. A grid was constructed to define the thermal refuge as a system of cells. Temperatures were monitored using remote temperature loggers and fish counts were conducted using daytime snorkelling. Most juvenile salmonids were observed moving into the refuge when main-stem temperatures exceeded 22-238C. Salmonids in the thermal refuge did not necessarily seek the coolest water, but were generally located in habitats commensurate with species-specific behavioural needs within their thermal tolerance range. Such ranges largely occurred within refuge areas. Variable meteorological conditions confounded observable biological thermal benefit to fish resulting from higher or lower main-stem flows. Thermal regime dynamics indicated that under the hydrological and meteorological conditions observed, higher flows from Iron Gate Dam showed some ability to change the structure of the refuge area. It appeared that without the thermal refuge, main-stem flows alone could not sustain the salmonid population because high water temperatures usually exceeded their published thermal tolerance limits.

Elevated stream temperature is a primary factor limiting the coho salmon (Oncorhynchus kisutch) population in California's Shasta River Basin. Understanding the mechanisms driving spatial and temporal trends in water temperature throughout the Shasta River is critical to prioritising river restoration efforts aimed at protecting this threatened species. During the summer, the majority of streamflow in the Shasta River comes from large-volume, cold-water springs at the head of the tributary Big Springs Creek. In this study, we evaluated the initial character of this spring water, as well as the downstream fate and transport of these groundwater inflows during July and August 2008. Our results indicated that Big Springs Creek paradoxically provided both cool and warm waters to the Shasta River. During this period, cool groundwater inflows heated rapidly in the downstream direction in response to thermal loads from incoming solar radiation. During the night time, groundwater inflows did not appreciably heat in transit through Big Springs Creek. These diurnally varying water temperature conditions were inherited by the Shasta River, producing longitudinal temperature patterns that were out of phase with ambient meteorological conditions up to 23 km downstream. Findings from this study suggest that large, constant temperature spring sources and spring-fed rivers impart unique stream temperature patterns on downstream river reaches that can determine reach-scale habitat suitability for cold-water fishes such as coho salmon. Recognising and quantifying the spatiotemporal patterns of water temperature downstream from large spring inflows can help identify and prioritize river restoration actions in locations where temperature patterns will allow rearing of cold-water fishes. Figure 1. Stream temperature monitoring sites located in the Shasta River Basin, California. Shasta River (SR) and Big Springs Creek (BSC) river kilometer locations (e.g. SR 44.03) and the downstream distance from groundwater spring sources located at the head of Big Springs Creek [e.g. (13.16)] are presented for each temperature logger WATER TEMPERATURE PATTERNS BELOW SPRINGS 443

Low instream flows and high water temperatures are two factors limiting survival of native salmon in California's Shasta River. This study examines the potential to improve fish habitat conditions by better managing water quantity and quality using flow and water temperature simulation to evaluate potential restoration alternatives. This analysis provides a reasonable estimate of current and potential flows and temperatures for a representative dry year (2001) in the Shasta River, California. Results suggest restoring and protecting cool spring-fed sources provides the most benefit for native salmon species from a broad range of restoration alternatives. Implementing a combination of restoration alternatives further improves instream habitat. Results also indicate that substituting higher quality water can sometimes benefit native species without increasing environmental water allocations. This study shows the importance of focusing on the limitations of specific river systems, rather than systematically increasing instream flow as a one size fits all restoration approach.