Sediment accumulation rates and volume in Pahranagat Wash above Arrow Canyon Dam in northern Moapa Valley, Nevada

An evaluation of sediment deposition rates and volume of impounded sediments in Pahranagat Wash behind Arrow Canyon dam in southeastern Nevada was done between 2016 and 2022. Data were collected and interpreted to address concerns by the Moapa Band of Paiutes and local historical preservation groups regarding the burial of culturally important sites by the impounded sediment deposited behind the dam. Sediment cores from two wells, drilled to depths that reflect the original stream-channel profile, and a third site drilled and sampled at a finer resolution, were analyzed radiometrically for lead-210 (²¹⁰Pb) and cesium-137 (¹³⁷Cs) isotopes. The analysis of the ²¹⁰Pb data yielded an overall estimated sediment deposition rate of 2.4 inches per year (in/yr). Using the ¹³⁷Cs data, the sediment deposition rate declined from 9.4 in/yr from 1934 to 1951 to 4.2 in/yr from 1951 to 1964 to 1.0 in/yr between 1964 and 2019.

Sediment volume was determined by defining boundaries using a 1-foot contour map generated using Unmanned Aerial Survey datasets and field observations. The volume calculation involved segmenting the study area based on the availability of sediment thickness data. The average sediment thickness in each segment was multiplied by the surface area of each segment to obtain the total sediment volume of 4.3×10⁷ cubic feet.