Modeling the relative morphodynamic influence of vegetation and large wood in a dryland ephemeral stream, Arizona, USA

Authors: SCAMARDO, J. - Colorado State University; Nichols, Mary; NELSON, P.A. - Colorado State University; WOHL, E. - Colorado State University

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/6/2022
Publication Date: 9/21/2022
Citation: Scamardo, J., Nichols, M.H., Nelson, P., Wohl, E. 2022. Modeling the relative morphodynamic influence of vegetation and large wood in a dryland ephemeral stream, Arizona, USA. Geomorphology. 147. Article 108444. https://doi.org/10.1016/j.geomorph.2022.108444.
DOI: https://doi.org/10.1016/j.geomorph.2022.108444

Interpretive Summary: Many studies have been conducted to investigate the impact of large wood in stream that flow year-round, but less attention has been paid to normally dry channels. Due to the flashy nature of flows in drylands, opportunities to directly investigate the influence of vegetation and woody debris in such channels is limited. We developed a model to investigate changes to channels and floodplains due to wood and vegetation in an ephemeral stream in southeastern Arizona, USA. Both large wood debris and vegetation effectively confined flow to the main, unvegetated channel area, which became wider and deeper over the course of a single moderate flood. Model results show that wood enhances channel change created by vegetation, resulting in ±0.1 to 0.3 m of additional scour or aggradation. We developed a conceptual framework that describes how large wood could impact sedimentation as well as the recruitment and growth of vegetation in ephemeral streams, thus contributing to the evolution of ephemeral stream shape over time.

Technical Abstract: Compared to perennial streams, studies investigating the impact of large wood on sediment transport and river corridor morphology in ephemeral streams are lacking. Due to the flashy nature of ephemeral flow regimes, opportunities to directly investigate the influence of wood in ephemeral channels are limited. Additionally, given prior studies showing a strong association between existing riparian vegetation and large wood deposition in ephemeral streams, the geomorphic impact of wood is entangled with that of vegetation. Here, we develop a hydro-morphodynamic model to investigate changes to channel and floodplain morphology due to wood and vegetation in an ephemeral stream in southeastern Arizona, USA. Three scenarios are modeled: the actual configuration of the river corridor; an experiment in which jams are removed; and an experiment in which vegetation is removed. Both large wood and vegetation effectively confined flow to the main, unvegetated channel, which became wider and deeper over the course of a single moderate flood. When isolating the impact of large wood, model results show that wood enhances channel change created by vegetation, resulting in ±0.1 to 0.3 m of additional scour or aggradation. The simulated removal of vegetation resulted in more channel change than the removal of wood alone, partially because vegetation occupies a much greater area within the stream corridor than large wood. We propose a conceptual framework where large wood could mediate sedimentation as well as the recruitment and growth of vegetation in ephemeral streams, contributing to the evolution of ephemeral stream morphology over time.