Introduction

Pahranagat Wash is a tributary to the Muddy River in southeastern Nevada (fig. 1), with a drainage area of approximately 994 square miles (mi²). Ephemeral runoff in Pahranagat Wash drains through a narrow channel in Arrow Canyon below Dead Man Wash (fig. 1). Discharge in the wash below the Arrow Canyon dam was measured between 1988 and 1993 at U.S. Geological Survey (USGS) surface-water gaging station 09415850 ( U.S. Geological Survey, 2020). The monthly mean discharge recorded during the period of record at this site includes a high of 0.92 cubic feet per second (ft³/s) in January and a low of 0.06 ft³/s in May, with several months of zero flow (table 1).

Floodwater in Pahranagat Wash is slowed at Arrow Canyon Dam (sometimes referred to as “the dam”) about 12.5 miles (mi) northwest of Moapa in Clark County, Nevada (fig. 1). The dam was constructed in 1934 by the Civilian Conservation Corps as a flood control and surface-water diversion (White, 2003) and was designed to store flood water and protect downstream property in Moapa Valley. The dam consists of a main structure built from locally quarried limestone approximately 35 feet (ft) tall and 30 ft wide at its base. A 2-ft-diameter outlet pipe extends through the base of the dam and allows drainage to a 32 ft wide, 6 ft-tall secondary dam and shallow rock apron that extends 33 ft downstream (White, 2003). Since its construction, periods of flooding that would have naturally transported sediments down the canyon have trapped fine-grained sediment behind the dam, filling the upstream pool to within approximately 7 ft of the dam’s crest. Sediments extend about 1 mile upstream in the basin and have covered culturally important sites, raising concern from the Moapa Band of Paiutes and local historical preservation groups. Additionally, because of sediment buildup behind the dam, it no longer functions as a flood control structure and is thus no longer protecting homes and tribal lands downstream from Moapa Valley (Federal Emergency Management Agency, 2011).

In 2019, the Bureau of Land Management (BLM) formed a cooperative agreement with the USGS to assess the volume and timing of sediment deposition behind Arrow Canyon Dam. The USGS evaluated the age, depositional rates, and volume of sediment in Pahranagat Wash by analyzing sediment cores from three data collection sites and conducting an unmanned aerial survey of the area behind Arrow Canyon Dam. The goal of this effort is to enhance understanding of the timing and magnitude of historical sediment accumulation and to assess the quantity of sediment behind the dam. This information will assist the BLM in determining which strategies are feasible for the management of the site.

Methods

To determine the age, accumulation rate, and volume of the sediment behind the dam, the sediment analysis included sediment core sampling, radiometric analysis, and elevation mapping. The sediment core sampling was analyzed at three different sites, with the objective to capture a detailed account of historical sediment deposition upstream from the Arrow Canyon Dam. This process was followed by a thorough radiometric analysis to assess the age and accumulation rates of the sediments, using isotopes to derive insights into sedimentation over time. Additionally, elevation mapping was used to create high-resolution topographic models that provided contour maps to estimate the volume of sediment.

Sediment Core Sampling

In July 2016, the USGS collected cores from two wells that were drilled to depths representative of the original channel profile (fig. 2). The original purpose of these cores was to allow the BLM to examine changes in the seed bank over time, but they were also used for this study. Sediment cores at well installation sites ACWEST (USGS site number 364453114483401) and ACEAST (USGS site number 364448114481001; fig. 1) were collected every 2.5 ft using a tractor-mounted, direct-push drill rig. Core depths were 14.0 ft at ACWEST, and 24.4 ft ACEAST. Coring stopped when gravels and cobbles were detected and downward progression with the coring bit slowed. Coarser sediments composed of sand, gravels, and cobbles were consistent with the natural channel deposits upstream from Dead Man Wash and were interpreted as the top of the pre-dam wash channel. Approximately 92 percent of the core from ACWEST well and 99 percent of the core from ACEAST well was recovered. Core loss-gaps primarily were caused by challenges during drilling, which included sediment being dislodged from the shoe because of increased turbulence and the flushing of unconsolidated sediments during sample retrieval (table 2). Additionally, some sections of core had been lost or discarded before this study began, resulting in additional gaps between samples.

Table 2.    

Depth and lithology of core from data collection sites at Pahranagat Wash upstream from Arrow Canyon Dam, Nevada.

[Core ID: "cap" sediment from top of core, "shoe" sediment from bottom of drill bit. Abbreviation: ID, identification]

Table 2.    Depth and lithology of core from data collection sites at Pahranagat Wash upstream from Arrow Canyon Dam, Nevada.

Core ID	Feet			Lithology
	From	To	Thickness
ACWEST
Cap	0.00	0.30	0.30	Clay
W-1	0.30	2.80	2.50
Shoe	2.80	3.00	0.20
W-2	3.00	5.50	2.50
W-3	5.50	8.00	2.50
W-4	8.00	10.50	2.50
W-5	10.50	13.00	2.50
W-6	13.00	14.00	1.00	Sand and gravel
ACEAST
cap	0.00	0.60	0.60	Clay
E-1	0.60	3.10	2.50
shoe	3.10	3.40	0.20
E-2	3.40	5.90	2.50	Damp clay
E-3	5.90	8.40	2.50	Clay
shoe	8.40	8.70	0.30
E-4	8.70	11.20	2.50	Gravel
shoe	11.20	11.50	0.30	Clay
E-5	11.50	14.00	2.50
shoe	14.00	14.30	0.30
E-6	14.30	16.80	2.50	Clay and sand
shoe	16.80	17.00	0.20
E-7	17.00	19.50	2.50	Sand and clay
shoe	19.50	19.80	0.30
E-8	19.80	22.30	2.50
E-9	22.30	23.70	1.40	Sand, clay, and gravel
E-10	23.70	24.40	0.70	Cobbles, gravel, and sand
Shallow sediment core site
S-1	0	0.5	0.50	Silt and clay
S-2	0.5	1	0.50
S-3	1	1.5	0.50
S-4	1.5	2	0.50
S-5	2	2.5	0.50	Sandy silt
S-6	2.5	2.8	0.30
S-7	2.8	3.2	0.40	Silt and clay
S-8	3.2	3.5	0.30	Sandy silt
S-9	3.5	4	0.50
S-10	4	4.3	0.30	Silt and clay
S-11	4.3	4.7	0.40
S-12	4.7	5	0.30
S-13	5	5.3	0.30
S-14	5.3	5.6	0.30
S-15	5.6	5.9	0.30
S-16	5.9	6.2	0.30
S-17	6.2	6.6	0.40
S-18	6.6	6.9	0.30
S-19	6.9	7.2	0.30
S-20	7.2	7.5	0.30
S-21	7.5	7.9	0.40
S-22	7.9	8.3	0.40
S-23	8.3	8.6	0.30
S-24	8.6	8.9	0.30
S-25	8.9	9.2	0.30
S-26	9.2	9.5	0.30
S-27	9.5	9.8	0.30
S-28	9.8	10.1	0.30
S-29	10.1	10.4	0.30
S-30	10.4	10.8	0.40
S-31	10.8	11.1	0.30

In February 2019, sediment from wells ACWEST (U.S. Geological Survey, 2019a) and ACEAST (U.S. Geological Survey, 2019b) were divided into vertical subsections, dried, sieved, and radiometrically analyzed at the USGS Menlo Park Sediment Radioisotope Lab. Results from the analysis prompted additional sediment sample collection and analysis. In April 2019, core from the shallow sediment core (SSC) site, between ACWEST and ACEAST, was collected to improve the depth resolution of the radiometric analysis (fig. 2). Samples were collected using a hand auger at intervals of 0.3–0.5 ft, with the differences in sample interval depending on either changes in lithology or changes in coring difficulty (table 2). The total depth of the SSC was 11.1 ft, and 100 percent of the core was retrieved.

Elevation Maps

The USGS created a topographic map of the Arrow Canyon area before the construction of Arrow Canyon Dam (U.S. Geological Survey, 1937). The benchmarks used to create the map were not clearly defined for the elevation contours to be used quantitatively in this study, but qualitatively, the map shows a steep-sided, flat-bottomed canyon.

In 2022, two Unmanned Aerial Survey datasets were collected and processed for the area around Pahranagat Wash above Arrow Canyon Dam; data are available in an accompanying data release (Wilson and others, 2025). Elevation and depth data collected at drill and sediment collection sites were used to determine total sediment volume.

Lidar data were collected using an uncrewed aircraft system on January 24, 2022 (Wilson and others, 2025). Nine check points were collected to compare to the lidar point-cloud data. The raw point cloud was filtered, and the bare-Earth points were used to produce a 1-meter resolution, bare-Earth digital elevation model (DEM). A 1-ft contour map was generated from the DEM (fig. 3).

Sediment Accumulation Rates

Concentrations of ²¹⁰Pb_ex activity with depth at core sites were graphically compared, averaged together, and fitted to an exponential decay curve for the SSC site (fig. 4). Values generally decrease with increasing depth, although there are several depths at which sediments with lower ²¹⁰Pb_ex overlay sediments with higher ²¹⁰Pb_ex. This pattern could be the result of older sediment from larger floods being deposited on top of more recent sediment from smaller floods. The CF:CS method was used to estimate an overall sedimentation rate of 2.4 inches per year.

The distribution of ¹³⁷Cs activities with depth observed in the three sediment collection sites also are similar. The following results for ¹³⁷Cs are presented for the SSC because it has the finest depth resolution; however, the patterns were consistent across all three sites (fig. 5). The first occurrence of ¹³⁷Cs activity above the detection limit is at 9.0 ft below land surface. The combined ¹³⁷Cs profile reaches maximum activity (interpreted as occurring approximately in the year 1964) at 4.4 ft below land surface, but there is a peak of similar magnitude at 5.7 ft below land surface (fig. 5). Because many atmospheric nuclear tests were carried out at the Nevada National Security Sites in southern Nevada, sediments in nearby areas may have separate ¹³⁷Cs peaks for each individual test (Rosen and others, 2012). The SSC does not have sufficient vertical resolution to detect all the atmospheric tests carried out at the Nevada National Security Sites, so the maximum value at 4.4 ft below land surface was presumed to represent the 1964 fallout peak.

Table 4 shows the sediment deposition rates calculated for 1934–51, 1951–64, and 1964–2019. The sediment deposition rate decreases over time. It is likely that a larger part of total sediment load transported in the first floods after dam construction was trapped behind the dam because of the large storage pool, resulting in a higher sediment deposition rate, whereas in later years, as sediment filled in the dam storage pool, less sediment deposition was possible, allowing floods to carry sediment downstream, which consequently reduced the deposition rate.

Sediment Volume

The sediment volume in the study area was estimated using indirect observations of flood deposits, primarily consisting of sand and clay, that developed after 1934 in the area between Dead Man Wash and Arrow Canyon Dam. The sediment volume was calculated by dividing the accumulated sediment into three subvolumes using the 1-ft contour map, sediment depth measurements from the ACWEST and ACEAST well sites, and elevation data from the DEM near Arrow Canyon Dam (fig. 6). The upstream area near Dead Man Wash was recorded as having zero depth. The measured depth of 4 ft at Arrow Canyon Dam was determined as the difference between the elevation below and above the dam, as indicated by the DEM. The depths between well sites are categorized into upstream, midstream, and downstream subvolumes. It is assumed that the depths within each subvolume change linearly from the upstream to downstream measurement points.

The upstream sediment subvolume represents the volume of sediment between well ACWEST and the elevation at which the sediment thins into cobbles and gravels. The area of this subvolume is defined as the area between the 2014- and 2020-foot contours, which correspond to the surface elevation of ACWEST and field observations of the easternmost impounded sediment, respectively. The average depth used to calculate the volume was 7 ft (the average of zero ft of impounded sediment at the upstream edge and 14 ft of impounded sediment at ACWEST).

The midstream sediment subvolume is between well ACWEST and well ACEAST. The area of this subvolume is defined as the area between the 2014- and 2009-foot contours, which correspond to the surface elevations of ACWEST and ACEAST, respectively. The average depth used to calculate the volume was 19 ft (the average of the 24 ft of impounded sediment at ACEAST and the 14 ft of impounded sediment at ACWEST).

The downstream sediment subvolume is between well ACEAST and the Arrow Canyon Dam. The area of this subvolume is defined as the area between the 2009-foot contour, which corresponds to the surface elevation of ACEAST and the dam. The average depth used to calculate the volume was 14 ft (the average of the 24 ft of impounded sediment at ACEAST and the estimated 4 ft of impounded sediment at the dam, where elevation drops sharply).

To determine sediment volume, the area of sediment behind the wash was divided into three distinct segments. For each of these segments, the average thickness of the sediment was measured and then multiplied by the corresponding surface area to determine the total volume for that segment (table 5). The cumulative total of the sediment volume across all three segments was 4.3×10⁷ ft³.

Conclusions

Using the constant flux: constant sedimentation method, the estimated overall sediment accumulation rate of the sediment behind Arrow Canyon Dam was estimated to be 2.4 inches per year (in/yr) from the excess lead-210 (²¹⁰Pb_ex) profile. Using the cesium-137 (¹³⁷Cs) chronology, the 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. The greater deposition rate in the years immediately after the dam was built indicate that even if all the sediment behind the dam was removed, the dam pool would refill quickly, likely because sediment-laden floodwaters would be impounded behind the dam.

The primary source of uncertainty in the ¹³⁷Cs chronology is the identification of the 1964 peak, which could be as deep as 5.7 feet (ft) below land surface. Using the 5.7-ft depth does not affect the 1934–51 deposition rate. The 1951–64 and 1964–2019 deposition rates change slightly but still decrease with time.

The sediment volume in the study area was determined by defining sediment boundaries with elevation maps and field observations. The volume calculation involved segmenting the study area into upstream, midstream, and downstream segments based on the availability of sediment depth 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. This total is the amount of sediment that would have to be removed to restore the area behind the dam to its original topography.

The largest source of uncertainty in the calculated sediment volume is the assumption that the area of the impounded sediment is the same at the surface as it is at depth. This assumption likely overestimates the calculated volume. Arrow Canyon was surveyed by the U.S. Geological Survey before construction. The resulting contour map shows that the canyon had steep walls and a broad, flat bottom, but the resolution of the map (20-ft contour intervals on the canyon walls) is insufficient to quantitatively evaluate the area of the canyon bottom. Further sediment coring to characterize the original canyon bottom surface would be needed to refine the calculated volume.