Geomorphic covariance analysis of thirty ephemeral streams in the South coast of California, USA.

Abstract:

An understanding of existing natural channel morphologies at hydraulic to segment spatial scales is crucial to designing river restoration projects that work synchronously with the morphological process assemblage present and evaluating the generalizability of geomorphological conceptual models. Despite the recognition that river corridors have hierarchically nested landforms, there is a paucity of research investigating the structure of multi-scalar topographic variability. For example, how do bed elevation and width pseudo-undulations relate as a function of spatial frequency? To answer this and related questions, 1-m scale digital elevation models (DEMs) were created from USGS airborne LiDAR data for thirty ephemeral river reaches in California’s South Coast region, across a representative range of catchment areas and channel-types. Standardized longitudinal channel width and detrended bed elevation profiles were extracted from the DEMs for a range of stages. Spatial series were analyzed individually and with respect to each other using coherence analysis and a relatively new framework called geomorphic covariance (GCS) analysis. GCS analysis facilitates a direct conceptual linkage between the channel topography present and models of morphological processes, notably the flow-convergence routing hypothesis for riffle-pool maintenance. Scale-independent, hierarchically nested fluvial landforms, defined in relation to the Flow-Convergence Routing hypothesis (i.e constricted pool, wide-bar) were identified for each reach and results were compared between reaches. Results were also compared in relation to regional geomorphic channel-type classifications, and in the context of morphological conceptual models.