Geomorphic Process Drivers in Unconfined, Alluvial River Corridors at Varying Temporal Scales

Abstract

Spatial and temporal variations in the fluxes of water, sediment, and wood drive changes in the three-dimensional structure of river corridors, which encompass the active channel(s), adjacent floodplain and riparian zones, and the underlying hyporheic zone. In natural, unconfined river corridors, the net effect of the interactions between the fluxes stored at least temporarily is to increase spatial heterogeneity, or patchiness in a river corridor. Unconfined river corridors—where valley widths exceed eight times the bankfull channel width—are especially dynamic: they integrate hillslope and fluvial processes, store and rework sediment and wood, and create physical and habitat diversity that supports ecosystem function and resilience. Unconfined valleys are also regions of concentrated human interactions with the landscape in terms of settlement, resource use, and natural hazards. Effective management of valley ecosystems depends on understanding physical and biological processes that maintain valley floor characteristics through time.

In the first part of our presentation, Sara will assess the alluvial dynamics of a formerly glaciated headwater valley. Unconfined valleys integrate hillslope and fluvial processes and are thus physically important sediment storage sites that preserve alluvial records of past landscape dynamics, including the transition from glacially- to fluvial-dominated processes. Using geologic mapping, ground-penetrating radar surveys, sediment coring, and geochronology, an unconfined portion of the South Fork Cache la Poudre River Valley, Colorado Front Range was investigated. The results are compared to other Colorado headwater valleys to broaden the foundation for understanding the processes that create and maintain alluvial fills, which is critical for effective management of these valleys facing ongoing development pressures.

In the second half of our presentation, Anna will demonstrate how pieces of downed wood and logjams serve as an integral part of the interactions among water, sediment, and the resulting form and function of unconfined alluvial valleys. Drawing insights from the Swan River in Montana’s Northern Rocky Mountains, she examines interactions and feedbacks between logjams and channel dynamics, and how these processes drive spatial heterogeneity across an unconfined river corridor. This work integrates field surveys, remote sensing, statistical modeling, and wood dating from radiocarbon and tree rings to capture the patterns, processes, and interactions of a dynamic system across multiple spatial and temporal scales.

Bio

Sara Rathburn is a Professor in the Department of Geosciences at Colorado State University. Sara is a fluvial geomorphologist interested in how rivers shape the landscape, especially in response to disturbances that abruptly increase sediment supply to rivers. Currently, with her students, Sara is investigating alluvial valley dynamics, the influence of vegetation on channel morphology, and the geomorphic context of river restoration and the use of beaver mimicry to promote channel-floodplain connectivity. 

Bio

Anna Marshall is an Assistant Professor in the Department of Geography & Sustainability at the University of Tennessee. Anna is a fluvial geomorphologist and received her PhD in Geosciences from Colorado State University. Her research focuses on river corridor dynamics and the interactions between geomorphic, hydraulic, ecological, and human processes. She is particularly interested in the role of unconfined river corridors within headwater catchments as they pertain to biophysical interactions, responses to disturbances, and efforts to restore processes and functions.