GIS-based Prioritization of Steelhead Stream Reaches for Spawning Gravel and Large Wood Augmentation

Year: 2023
Presenter/s: Steven McNeely
Symposium Session: 2023 - 09 - An Open Toolbox: Tools and Assessments to Tackle Complex Questions
Topics covered: modeling, sediment transport, and wood


In streams that support salmonid populations, reservoir operations and other anthropogenic activities reduce the natural supply and transport of gravel and large wood, which adversely impacts habitat quality. In Santa Clara County, California, the Santa Clara Valley Water District, also known as Valley Water, administers the Safe, Clean Water and Natural Flood Protection Program, which includes projects focused on restoring and maintaining populations of the federally threatened Central and South-Central California Coast distinct population segments of steelhead (Oncorhynchus mykiss). In support of these efforts, AECOM conducted a study of steelhead streams in the county to identify priority locations for spawning gravel and large wood augmentation. We developed GIS workflows to organize and analyze existing spatial data sets to divide the study streams into reaches. Data used to delineate reaches included steam network properties, limits of anadromy, fish functions and values, bed and bank conditions, and FEMA floodplains and floodways. We then prioritized the delineated reaches for spawning gravel and large wood augmentation based on a list of criteria developed in coordination with Valley Water that included 1) source watershed disconnection, 2) remaining source protection, 3) likelihood to improve geomorphic function, 4) proximity to sediment sink, 5) likelihood to improve steelhead habitat, 6) risk of increased flooding, 7) ease of implementation and 8) fish passage impediments. We developed a complex weighted scoring model to produce a reach score for each of these criteria, as well as total reach scores for both spawning gravel and large wood augmentation priority. The weighted scoring model is carried out in GIS using nine hard-coded Python scripts, which can be run directly within ArcMap’s Python command window. Workflow documentation included a step-by-step explanation of all GIS analysis, with references to GIS layers, geoprocessing steps/parameters, and Python scripts to run. This documentation will allow Valley Water to modify any part of the process or update reach prioritization scoring when newer spatial data become available. A similar process of prioritizing stream reaches for spawning gravel and large wood may be useful to stewards of salmonid supporting streams elsewhere on the west coast, including the Pacific Northwest.