Analytical Approaches to Determine Instream Flow Needs for Salmonids and River Ecosystems

by Darren Mierau, California Trout

Instream flow studies share many common traits, the first of which is the need to identify streamflows that protect basic ecological attributes (e.g., processes, populations) of rivers and streams. In the face of competing demand for water resources, instream flow studies are often contentious and highly scrutinized. Studies typically involve some level of data collection to quantify instream conditions (e.g., water temperature) and relationships between physical conditions and water quantity (e.g., habitat-flow curves), evaluation of water supply availability, and often negotiation to establish instream flow requirements.

Perhaps the most challenging step in developing instream flow recommendations is the analysis and integration of disparate data sets to identify a flow regime that balances competing ecological attributes (e.g., fluvial processes, riparian ecology, water temperature, and physical habitat structure). The integration stage of instream flow analysis often must address spatial and temporal variability in flow and habitat conditions, and consider habitat capacity (spatial habitat factors), biological productivity (growth rates, inter-specific competition, survival), and life history diversity (phenotypic and genotypic). Additionally, flow studies must conduct this analysis in a transparent manner that allows non-technical stakeholders to participate in decision-making. Ultimately the instream flow needs must be weighed against diversion needs and hydrograph manipulation necessary to meet existing and future water demand.

Currently numerous instream flow studies or programs are being implemented in California, several of which have employed new approaches to data analysis and integration. Setting aside the often-debated topic of instream flow methods, this workshop will (1) offer a set of principles to guide flow study data analysis and integration, (2) describe several flow studies currently being implemented or just completed that utilize different analytical approaches, and (3) allow instream flow practitioners and others an opportunity to discuss concepts and approaches that may contribute to improving the application of study results.