276-4 At the Intersection of Geomorphology, Ecology, & Hydrology: Feedback Mechanisms Resulting in Contrasting Morphodynamic Responses in Dryland Riparian Zones

Session: Natural and Urban Channel Responses to Disturbance, Management, and Restoration

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Abstract:

Drylands make up over 40% of landscapes on Earth, yet ephemerally flowing streams in semi-arid to arid climates are arguably less well studied than their perennial counterparts. The ability to quantify feedbacks between vegetation, sediment characteristics, and channel geometries for these regions specifically is crucial to assessing flood risks and informing river management in the face of perturbations. Reaches of groundwater-fed springs along flash flood-dominated channels near the Henry Mountains of Southeastern Utah provide a unique opportunity to isolate variables, mechanisms, and feedbacks that mutually control alluvial channel morphodynamics and riparian vegetation distributions. In this landscape, vegetation density and species distribution naturally vary over distances of 10s to 100s of meters. Our previous work at this site empirically demonstrates that increasing vegetation abundances on channel banks and bed can drive complex and contrasting responses in channel morphology, including both narrowing and widening. This work aims to quantitatively understand the magnitudes and directionality of specific feedback mechanisms between vegetation growth, surface and groundwater hydrology, sediment transport, and channel morphology, and to evaluate how these feedbacks regulate the fluxes of water and carbon in dryland riparian zones. We hypothesize that in a narrowing regime, increasing bank vegetation enhances bank cohesion and reduces bank erosion, enhancing sediment entrainment from the channel bed. This works to continue the preferential establishment of bank-only vegetation. Conversely, channel widening occurs when vegetation is able to establish on both the bank and bed. Increased cohesion, water retention, and reduced erodibility on both the bank and bed creates a system in which the bank strength is comparable to that of the bed, inducing widening. In future work, we will combine field measurements, numerical modeling, and recent advances in machine learning to evaluate these hypothesized feedbacks, and to generalize our results through explorations of a broader range of parameter space.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9940

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