REDEFINING THE EPHEMERAL STREAM FLOODPLAIN: IDENTIFICATION AND IMPORTANCE OF FLOOD ZONES IN DRYLANDS

Although research on hydrologic, geomorphic, and biotic floodplain processes has largely focused on perennial river corridors, ephemeral streams may create similarly functional floodplain units. Variably inundated floodplains along perennial streams are known to host a high density of ecosystem functions, including the attenuation of downstream fluxes and provision of habitat to diverse flora and fauna. In contrast, variable inundation characteristic of non-perennial channels complicates delineating and understanding the importance of ephemeral stream floodplains. Here, we review the literature on dryland ephemeral river corridors in order to identify: (1) common definitions and styles of ephemeral stream floodplains; (2) the hydrologic, geomorphic, and biotic functions hosted in the floodplain unique from uplands or channels; and (3) current and future anthropogenic and natural stressors to floodplain functions. Although active floodplain definitions follow criteria commonly used in perennial river corridors, the extent of ephemeral stream floodplains is typically defined by higher recurrence interval floods. A set of floodplain styles emerge from the literature dependent on confinement and the presence of channelized streamflow. Functions related to attenuation and storage are typically concentrated in unconfined and channeled floodplains. The temporary storage of sediment and sub-surface water in ephemeral stream floodplains make them hotspots for biogeochemical cycling and hosts to richer, denser, and more diverse vegetation communities compared to surrounding uplands. Many functions of ephemeral stream floodplains are also found in perennial counterparts, but flashy flow regimes and high sediment loads in ephemeral streams can potentially impact rates and magnitudes of comparable processes and functions. Future research needs to incorporate a holistic view of the ephemeral river corridor – including channels and floodplains – in order to better understand current and future fluvial processes and cascading impacts in drylands globally.