Identifying key geomorphic parameters characterizing ecohydraulic responses of river channels using River Builder

Geometric modeling of river topography is a method of synthesis wherein specific 2D geometric elements of river topography, such as the bed profile, cross section, and channel planform, are mathematically modeled in isolation and then combined to produce a 3D heightmap. River Builder is an open-source, free, public python3 package able to create a complex synthetic channel with multiple parameters (e.g., valley slope, inner/outer channel lateral/vertical offset) and functions (e.g., inner/outer channel width variation, thalweg elevation). We created synthetic river archetypes to evaluate the ecological functionality of geomorphically distinct portions of the stream network across a range of flow regimes to develop functional flow criteria. Furthermore, we examined the roles of bed roughness, thalweg elevation and width variation in replicating real channel terrain and its ecohydraulic response. The study area was the high-relief South Fork Eel River network in coastal northern California. For 11 reaches we compared high-resolution surveyed terrain with synthetic terrain models, their probability distributions of hydraulic variables, and flow-habitat area curves. For most sites, thalweg elevation variation was the key factor characterizing the ecohydraulic responses of a system whereas bed roughness was a major parameter in the cobble-boulder, cascade/step-pool channel type.