128-13 Coupling Floodplain Nourishment with Sediment Transport and Deposition in a Semi-Arid Region, Elephant Butte Reservoir, New Mexico

Abstract:

Floodplain nourishment is driven by flooding, during which sediment and nutrients are delivered and stored across the floodplain. In semi-arid and arid environments, flashy hydrology and seasonal variations in discharge strongly influence whether deposited sediment is preserved or desiccated and reworked. Dams further modify these environments by retaining sediment upstream of impoundments and altering downstream sediment supply. The routing, deposition, and preservation of fine sediment across semi-arid floodplains remain poorly understood. This study investigates how seasonally driven flood events are recorded within the stratigraphy of a crevasse splay at the northern reach of Elephant Butte Reservoir (EBR), New Mexico, on the Rio Grande floodplain.

To understand sediment deposition across the crevasse splay, four L-shaped trenches were excavated 10, 20, 30, and 40 meters from the active bank, perpendicular to the primary flow direction of the Rio Grande River, and one sediment core was collected 130 meters from the riverbank. An advection-settling model was used to compare observed stratigraphy with predicted spatial variation in grain-size distribution across the crevasse splay. The trenches expose sand-dominated beds with bidirectional flow structures and erosional contacts. The sediment core contains alternating mud and sand beds separated by sharp contacts. These results indicate that the crevasse splay preferentially preserves coarse sediment near the channel, whereas fine sediment is readily reworked by subsequent floods or aeolian transport following desiccation, limiting its long-term retention within the proximal splay. Floodplain nourishment in this semi-arid system is therefore governed by flood stage and inundation frequency, which determines whether fine sediment reaches areas where it can be buried and preserved before desiccation and reworking to remove it.  

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