Drivers of Geomorphic Heterogeneity in Unconfined Non‐Perennial River Corridors

Abstract

River corridors along non‐perennial stream networks provide diverse physical and ecological functions that are thought to be related to the spatial and temporal variability of geomorphic units, also known as geomorphic heterogeneity. While studies on the characteristics and drivers of geomorphic heterogeneity have been developed in perennial streams, similar studies in ephemeral streams are lacking. Given the ubiquity of non‐perennial streams globally, we aim to answer questions regarding the magnitude and drivers of geomorphic heterogeneity in ephemeral river corridors as well as how geomorphic unit assemblages reflect processes related to flood disturbance. Geomorphic units were mapped in 30 unconfined river corridors within six non‐perennial watersheds in Utah and Arizona, USA. Landscape heterogeneity metrics—Shannon's Diversity Index, Shannon's Evenness Index, and patch density—were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, and proxies for flood disturbance. While heterogeneity positively correlated with metrics for morphology and disturbance, statistical models suggest that morphologic context, particularly river corridor width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long‐term temporal heterogeneity are needed.

Recognizing the ephemeral stream floodplain: Identification and importance of flood zones in drylands

“…In turn, geomorphic heterogeneity can influence the hydroperiod across the floodplain, which may influence the rate and timing of infiltration (e.g., Schilling et al, 2020). The flood regime additionally determines certain aspects of the ephemeral stream floodplain, such as rates of infiltration (Schilling et al, 2021), the density and diversity of vegetation (Stromberg et al, 2005(Stromberg et al, , 2009) and potentially the rates at which geomorphic units form and evolve (Scamardo et al, 2023). Many similar connections have been made or hypothesized throughout the literature (see Sections 3-5), resulting in a web of interconnected functions and attributes (Figure 8).…”

“…At the reach scale, ephemeral stream floodplain extents have also been delineated using field-based mapping reliant on topographic surveys and vegetative indicators. A steeper slope leading to a higher terrace or upland surface has commonly been used to topographically identify the lateral extent of the ephemeral stream floodplain both in the field (e.g., Bagstad et al, 2006;Patton & Boison, 1986;Reynolds & Shafroth, 2017;Ringrose et al, 2014;Scamardo et al, 2023) and using contour maps (e.g., Maxwell et al, 2021).…”

“…Lower stream powers in unconfined ephemeral stream floodplains can result in thicker alluvium compared with confined settings (Sutfin et al, 2014) and more diverse topography (e.g., Grenfell et al, 2008;Scamardo et al, 2023), which can result in greater variability in the hydroperiod and flow depth across the stream corridor compared with other styles of channelled floodplains. The accommodation space and associated temporary storage provided by unconfined settings also allow for the formation of wetlands (perennial, seasonal and ephemeral), including backswamps, discontinuous ponds and oxbow lakes (e.g., Grenfell et al, 2008;McCarthy et al, 2011;Stromberg et al, 2009;Tooth & McCarthy, 2007).…”

“…Lateral and vertical accretion and subsequent erosion can drive the creation of new geomorphic units on ephemeral stream floodplains, producing geomorphic diversity similar to perennial floodplains (Rabanaque et al, 2022;Scamardo et al, 2023). The creation of geomorphic units and complex topography could potentially drive variations in hydroperiod across the ephemeral stream floodplain, thus potentially creating heterogeneity in habitat and infiltration.…”