RAPID: The effects of extreme drought on sediment transport and deposition in water-supply reservoirs

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The southwestern U.S. is experiencing an extreme drought that is reducing flow in river systems and producing record low levels in water-supply reservoirs. For example, the Elephant Butte Reservoir on the Rio Grande River in New Mexico was only 5.6% full as of October, exposing a large wedge of sediment that has extended an additional 1 km into the reservoir during the summer of 2021. During the drought, flash floods are the principal means by which water enters the reservoir. Flash floods bring sediment-laden flows that can accumulate in the reservoir and reduce the capacity further. This project will collect time-sensitive data to assess impacts of the extreme drought on water budgets, river sediment transport, and reservoir sedimentation. The project will train one graduate student in the integration of field, laboratory methods, and numerical modeling. The research has applications to similar processes occurring in coastal regions and will help in making complex decisions on water release and management of wetlands in western river corridors. The severe drought in the Rio Grande River basin has caused a delta to emerge at the entrance into the Elephant Butte Reservoir. During the drought year of 2021, spring snowmelt was subdued, and flash floods from storm events became the predominant mechanism for sediment transport into the reservoir. These flash flood-derived sediments accumulate upon entering reservoirs to form a lacustrine delta, which unlike marine deltas, typically experience hyperpycnal flows. Initial research during a monsoon-driven flash flood in July 2021 indicate that heightened sediment and organic carbon transport affected the geomorphology and ecology during the event. This RAPID project will identify the mechanisms driving flash flood sediment transport and morphodynamics in arid fluvio-deltaic systems and determine how extreme drought amplifies these processes. The research will also numerically model the Elephant Butte Reservoir deltaic system to evaluate geomorphic changes in the subaqueous delta driven by highly sediment-laden flash floods, which will also inform the carbon sequestration potential of such rapidly prograding deltas. The proposed research involves two field campaigns in the winter and spring of 2022 to collect water and bed material samples, channel geometry, velocity, and water level measurements, images of the delta front, and casts of a conductivity, temperature, and depth sensor. The data will be incorporated into the hydrodynamics and sediment transport model Delft3D to simulate sediment transport and deposition processes during a flash flood at high resolution, which has never been done in a reservoir setting.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.