FLASHFLOODS CAUSE SUBAQUEOUS DELTA EVOLUTION AT DROUGHT-STRICKEN ELEPHANT BUTTE RESERVOIR, NEW MEXICO

Reservoirs across the American West are infilling with sediment and decreasing in water storage capacity. This study evaluates sediment transport and deposition within the Elephant Butte Reservoir (EBR) delta on the Rio Grande River, New Mexico. This reservoir is characteristic of the interior west due to its arid source region climate and proclivity to monsoon-driven flashfloods. In recent years, extreme drought and low reservoir levels exposed much of the EBR delta, revealing abandoned lobe complexes and channels that record how the delta evolved. Through field observations and measurements of modern sediment transport, we find that hyperpycnal flows generated by flashfloods primarily drive delta growth. Here we characterize sediment transport and depositional patterns in the EBR delta using field measurements of suspended sediment concentration (SSC), organic matter content, and conductivity, temperature, and depth (CTD casts) over 2021-2022. We put this drought year into perspective of historical Rio Grande discharge and EBR water level records, used in combination with remote sensing to evaluate the impact of highly variable flow on arid delta evolution. We conducted three field surveys, one during each phase of the hydrological year (winter low flow, spring snowmelt, and summer monsoon). During a monsoon-triggered flashflood in July 2021, we measured a maximum SSC in the Rio Grande of 46.7 kg/m³—well above the threshold for hyperpycnal flow generation in freshwater (1 kg/m³). Plunging flow conditions during spring snowmelt were recorded by CTD casts and water column samples collected at the delta front, which display strong density and SSC stratification. Hence, hyperpycnal flows likely develop at the river mouth during both prolonged (spring snowmelt) and short-lived (summer monsoon) floods, while no evidence of flow stratification was found during winter low flow conditions. Plunging hyperpycnal flows rapidly bury organic carbon on the reservoir bottom similar­ to a turbidity current. In this extreme drought year of 2021 and 2022, indeed the deposits are sequestered into relatively deep parts of the reservoir. This unique dataset suggests that hyperpycnal flows into EBR are easily generated by sediment-laden floods on the Rio Grande, controlling the delta’s resultant morphology and evolution.