Geospatial organization of fluvial landforms in a gravel–cobble river: Beyond the riffle–pool couplet

Abstract

Morphological units (MU) are landforms with distinct local form–process associations at ~1–10 channel widths scale that may be the fundamental building blocks describing the geomorphic structure of a river. Past research has disproportionately focused on the two MUs of pool and riffle, conjecturing that they are the central linked couplet in the process–form association. The goal of this study was to delineate and map spatially explicit fluvial landforms in two-dimensional planview within a gravel–cobble bed river using two-dimensional hydrodynamic delineation and then to statistically examine MU geospatial patterns for indicators of deterministic geomorphic control. This procedure is not discharge-dependent like mesohabitat methods, but gets at the geometry of underlying landforms. Statistical testing confirmed that eight delineated in-channel MU types comprise a complex and diverse channel morphology in which pools and riffles are not directly coupled. Specifically, gravel–cobble river channels (1) exhibit nonrandom spatial organization of their longitudinally and laterally variable landform morphology; (2) consist of a variety of MU types, not just pools and riffles; and (3) show distinct MU collocations and avoidances, with riffles linked to chutes and runs, while pools are linked to slackwaters and glides. Planview MU delineation with two-dimensional hydrodynamic modeling provides a ‘bottom-up’ approach to understanding and linking channel morphology with ecosystem services and geomorphic processes and is being used to guide river management and rehabilitation strategies.