Stream flows for salmon and society: managing water for human and ecosystem needs in Mediterranean-climate California
This dissertation addresses the complex relationships between human water demands and ecosystem water needs in the Mediterranean-climate region of northern California. Through a combination of long-term ecological data analysis, hydrologic modeling, and field studies of ecological-flow relationships, the research presented examines the challenges and opportunities for managing water to sustain freshwater ecosystems, with a particular emphasis on the environmental flow requirements of steelhead trout (Oncorhynchus mykiss). In coastal California watersheds, stream flows are increasingly impacted by small-scale withdrawals from agriculture and residential water users. While the effects of large dams have been extensively-studied, relatively little is known about the cumulative impacts of small-scale water-management practices on freshwater ecosystems. Analysis of a long-term record of fish count data revealed a strong positive relationship between stream flow and the over-summer survival of juvenile salmon and provided evidence that insufficient stream flows during the spring and summer rearing period has become an important limiting factor to threatened steelhead trout populations. These finding suggest that changes in water use practices that maintain dry season flows are critical for salmon population recovery.
A reduction in dry season water diversions could potentially be achieved by increasing local storage capacity in ponds, which could be filled during the wet season when there is greater water availability. However, increased water storage has the potential to impair winter flows, which are important for fish passage and spawning and the maintenance of habitat heterogeneity. To quantify the trade-offs between alternative water management strategies, an integrated management framework is introduced that simultaneously considers the temporal and spatial dynamics of flow regimes, and the water needs of both human and natural systems. The management framework relies on a watershed hydrologic routing model, which is useful for representing the temporal and spatial distribution of water availability and predicting the impacts of water diversions across the stream network. An examination of the potential impacts of water management practices in a Sonoma County, California watershed demonstrated that the location and size of water storage projects influences the magnitude and duration of stream flow impacts. The findings also illustrated trade-offs between environmental flow protections and the ability for water users to meet storage demands.
Managing stream flows for ecological benefits requires an understanding of flow-habitat relationships at scales relevant to individual organisms. To assess the flow needs of adult salmon for their upstream migration, a new approach was developed for mapping stream channel topography and modeling flows in relation to fish passage suitability. The approach relies on high-resolution measurements of stream channel topography derived from terrestrial LiDAR surveys. A two-dimensional hydraulic model was then used to simulate flows in several survey reaches and identify the minimum flow required to maintain a continuous migration path of suitable passage depths for adult salmon. The modeling approach is compared to a regional formula used by State of California resource agencies to estimate salmon passage flow needs. While the results were similar, improvements to State's regional formula could be made by explicitly incorporating a variable describing channel typology.
In sum, these studies quantify the response of a threatened salmon species to flow variation, illustrate the relationships between water management and ecologically-important flow dynamics, and establish a new approach for evaluating biological flow thresholds, all of which are critical for improving the management of freshwater ecosystems in Mediterranean-climate regions.