Comparing RIVER2D and HEC‐RAS Hydraulic Parameters Generated for Coho Salmon and Steelhead Rearing Flow‐Habitat Indices in Mark West Creek, Sonoma County, California

ABSTRACT The manuscript is a proof of concept to use HEC‐RAS, in place of River2D, to estimate water depth and velocity to compute flow‐habitat indices. The manuscript compares salmonid flow‐habitat index results using the two‐dimensional (2D) model programs, River2D and HEC‐RAS. Data collection and modeling were completed for three different study sites in Upper Mark West Creek, a tributary to the Russian River in Sonoma County California. Upper Mark West Creek supports Coho Salmon and steelhead including juvenile rearing over summer. Unlike River2D, HEC‐RAS does not compute the flow‐habitat index, area weighted suitability (AWS) internally. A Python script was used to automate computation of AWS from depth and velocity rasters exported from HEC‐RAS. Roughness was varied spatially in both programs to calibrate simulations. Model outputs lead to similar AWS curve patterns between the two models, generally giving the same peak of curve flow, but the amplitude of the curves differed by model and species. Validation results indicated that River2D and HEC‐RAS predictions of depth and velocity were more closely correlated with each other than with the field data. HEC‐RAS may be a sufficient alternative to River2D to simulate depth and velocity to compute AWS. However, the accuracy of both platforms was limited by the overall ability of 2D modeling to predict depths and velocities in complex streambed topography where turbulence and vertical mixing are present.