PATTERNS AND DRIVERS OF FLOODPLAIN HETEROGENEITY IN DRYLAND EPHEMERAL WATERSHEDS ACROSS THE SOUTHWEST USA

Floodplains adjacent to ephemeral streams are important zones of groundwater infiltration, nutrient cycling, and habitat in dryland environments. Ecosystem benefits of ephemeral stream floodplains are thought to be correlated to high geomorphic heterogeneity. However, while studies on the range and drivers of geomorphic heterogeneity are emerging in perennial streams, similar studies in ephemeral streams are lacking. Questions remain, such as: how does river corridor heterogeneity vary throughout ephemeral watersheds, and how do floodplain morphology and hydrologic variability influence heterogeneity? To address these questions, we mapped geomorphic units and quantified heterogeneity using field surveys and aerial imagery within 30 river corridor reaches across six ephemeral watersheds in Utah and Arizona, USA. Additionally, we quantified variables that may potentially constrain or drive heterogeneity in ephemeral river corridors, including floodplain shape, grain size, the number and density of large wood jams, and flood recurrence interval. Because most of the streams are ungauged, channel change over the last decade was used as a proxy for flood disturbance, assuming that a greater magnitude or frequency of flooding will result in a more dynamic channel and less overlap between the modern channel and channels mapped from past aerial imagery. Floodplain heterogeneity, as quantified via Simpson’s Diversity Index, positively correlated with floodplain width and frequency of large wood jams, and negatively correlated with channel overlap between the modern and 10-year historical channel. Results show that wider floodplains are disproportionately important areas of ecosystem benefits in ephemeral watersheds. Similar to perennial streams, results also highlight potential positive feedbacks between large wood jams and river corridor complexity. Finally, channel change driven by flash flooding significantly increases geomorphic heterogeneity within floodplains. Study findings emphasize that geomorphic complexity and habitat in dryland ephemeral streams may be susceptible to climate change and alterations in flow regime, but also highlight that conservation of wide, unconfined floodplains as well as restoration using large wood could improve future management of ephemeral streams.