| Water temperature exerts powerful influences on biotic and abiotic processes within a stream, determining the distribution of aquatic organisms on both coarse and fine scales. This dissertation examines stream temperature patterns and their influence on the distribution of salmonids in the Garcia River watershed in Mendocino County, California. Chapter 1 reviews three broad, interconnected themes within the literature: stream temperature dynamics, anthropogenic origins of elevated stream temperatures, and stream temperature impacts on salmonid species. The following chapters expand on these themes.; Chapter 2 presents a modified time series and regression approach to predict mean and maximum water temperatures from air temperatures. By decomposing daily stream temperatures into an annual component for a day t and a residual component that responds to fluctuations in air temperatures, I was able to predict daily mean and maximum water temperatures and seven-day moving averages with good to excellent results.; Chapter 3 uses field-based measurements to quantify relationships between stream temperature, channel morphology, and riparian conditions in 15 tributary streams, then models the effect of changes in shading parameters alone. Using maximum weekly average temperature (MWAT) and maximum weekly maximum temperature (MWMT) as response variables, the model predicted no statistically significant effects of increasing canopy closure from current levels to 75% in any of the tributary streams. However, when mean and maximum water temperatures from the first day of each summer month were analyzed, a 50% reduction in available solar radiation produced significant, although slight, temperature decreases.; Chapter 4 examines temperature records, stream- and basin-scale habitat factors, and the distribution of coho salmon (Oncorhynchus kisutch ) and steelhead (O. mykiss) in the Garcia watershed to determine whether temperature alone explains the observed pattern of presence and absence. Using logistic regression techniques, I found that coho presence was best explained by a combination of low maximum weekly average temperatures and a high volume of large woody debris. Steelhead presence was best explained by a combination of low gradients and a high percentage of reach length occupied by pools. Temperature was not a factor in the distribution of steelhead, even though inland temperatures frequently exceed published tolerance levels. |
