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Geomorphic analysis of 35 ephemeral river reaches with variable channel morphologies in the South Coast, California, USA.


During the last decade, 1-m resolution topo-bathymetric digital elevation models (DEMs) have become increasingly utilized within fluvial geomorphology, but most meter-scale geomorphic analyses have only been conducted on one to a small handful of river reaches. While such analyses have contributed greatly to our collective understanding of river discharge-topography interactions, which is applicable in both river restoration design and environmental flow regulation contexts, their generalizability across a range of river types remained largely unevaluated. This study assessed the dominance of a single hydro-morphodynamic mechanism, flow convergence routing, in 35 ephemeral rivers divided among 5 channel types in California's South Coast region. To do so, we conducted geomorphic covariance structure (GCS) analysis on longitudinal standardized width (Ws) and standardized, detrended bed elevation (Zs) spatial series from 1-m resolution DEMs to explore the stage-dependent nature of fluvial landforms. We compared and contrasted results among different river types and found that in all river types there are coherent, multi-scalar structures of longitudinal fluvial topography. We find that width undulation driven flow convergence routing is a broadly relevant channel altering mechanism, however, it’s relationship with water stage height differs between river types. For example, in partly confined riffle-pool rivers, resilient width undulations at a bankfull discharge appear to control the degree to which riffle-pool morphology is self-maintaining. But in mountainous confined river reaches, width’s relationship with inferred sedimentary dynamics increases somewhat linearly with water stage height, potentially as a function of the sediment size distribution.

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