CARBON SEQUESTRATION AS A CONSEQUENCE OF HIGH SEDIMENTATION AND DROUGHT AT ELEPHANT BUTTE RESERVOIR, NEW MEXICO

In response to extreme droughts in the Western United States, reservoirs have been constructed to sustain irrigation and municipal water supplies. However, these reservoirs lose up to 2% of their capacity annually due to high sedimentation rates, driven by the sparse vegetation and easily erodible sediment in semiarid catchments. Along with sediment, reservoirs sequester organic matter (OM), burying ~0.15 PgC/year, a rate approaching that of oceanic carbon burial. This study evaluates sediment accumulation and carbon sequestration of a delta building into Elephant Butte Reservoir (EBR), on the Rio Grande River in New Mexico. The EBR delta exhibits rapid sedimentation driven by annual monsoon flooding and spring snowmelt, with an average suspended sediment concentration of 1.548×10−4">1.5×10⁻⁴. As sediment accumulates, OM is sequestered, potentially enabling the reservoir to operate as an organic carbon (OC) sink. Lake level determines the flooding extent of the EBR delta. During high water levels, the delta plain is inundated, and deposition of sediment and OM occurs by overbank sedimentation and is focused upstream. During low base-level conditions, more of the delta plain is subaerial, exposing shallow sediment and OC to the atmosphere. Depending on the burial depth and degree of sediment disturbance following deposition, OM may be converted to CO₂ and released to the atmosphere. We evaluate the potential for OC storage and release at EBR through remote sensing and field observation. We estimate the frequency and periodicity of inundation within EBR by calculating the wetted frequency, using the Normalized Difference Water Index applied to images obtained from Landsat missions 4, 5, 7, and 8. We obtained sediment grab samples and excavated trenches (up to 60 cm deep) across a range of wetted frequencies in the EBR delta. We used loss on ignition to estimate organic matter content and used a laser particle size analyzer to determine grain size distribution. We expect carbon content to correlate with wetted frequency, wherein a less inundated sampling area is expected to coincide with less carbon content in the sample because inundation promotes deposition and minimizes remobilization of sediment and organic carbon. Understanding the impacts of inland reservoirs on OC is essential to understand its effects on the global carbon cycle fully.