Research Paper GEOSPHERE Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of 10Be GEOSPHERE; v. 11, no. 4 Andrew J. Cyr1, David M. Miller1, and Shannon A. Mahan2 1U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA doi:10.1130/GES01134.1 2U.S. Geological Survey, Box 25046, MS 974, Denver Federal Center, Denver, Colorado 80225, USA 6 figures; 2 tables; 5 supplemental files ABSTRACT climatic drivers (e.g., Enzel et al., 2003, and references therein; Miller et al., CORRESPONDENCE: [email protected] 2010; Kirby et al., 2012; Lyle et al., 2012; Antinao and McDonald, 2013). How- The paleohydrology of ephemeral stream systems is an important con- ever, placing constraints on the paleodischarge of fluvial systems remains a CITATION: Cyr, A.J., Miller, D.M., and Mahan, S.A., straint on paleoclimatic conditions in arid environments, but remains difficult difficult problem. 2015, Paleodischarge of the Mojave River, south- western United States, investigated with single- to measure quantitatively. For example, sedimentary records of the size and Reconstructions of the paleohydrology of arid regions, particularly south- pebble measurements of 10Be: Geosphere, v. 11, extent of pluvial lakes in the Mojave Desert (southwestern USA) have been western North America, are often based on geologic and geomorphic evidence no. 4, p. 1158–1171, doi:10.1130/GES01134.1. used as a proxy for Quaternary climate variability. Although the delivery mech- of the size and extent of alluvial fans (e.g., Wells et al., 1987; Bull, 1991; Harvey anisms of this additional water are still being debated, it is generally agreed and Wells, 1994; Harvey et al., 1999) or eolian deposits (e.g., Lancaster and Received 7 October 2014 that the discharge of the Mojave River, which supplied water for several Pleis- Tchakerian, 2003), the stable isotopic records in speleothems (e.g., Winograd Revision received 30 April 2015 Accepted 15 June 2015 tocene pluvial lakes along its course, must have been significantly greater et al., 2006; Wagner et al., 2010), or lacustrine sedimentary records, particularly Published online 15 July 2015 during lake highstands. We used the 10Be concentrations of 10 individual of pluvial lakes (e.g., Enzel and Wells, 1997; Enzel et al., 1992, 2003; Wells et al., quartzite pebbles sourced from the San Bernardino Mountains and collected 2003). A pluvial lake is a closed (endorheic) basin that fills with water during from a ca. 25 ka strath terrace of the Mojave River near Barstow, California, to wetter climate periods. Observations of pluvial lake sedimentary records in the test whether pebble ages record the timing of large paleodischarge of the Mo- Mojave Desert, southern California, indicate that the highest lake levels, and jave River. Our exposure ages indicate that periods of discharge large enough therefore wettest climate conditions, were generally contemporaneous with to transport pebble-sized sediment occurred at least 4 times over the past glacial stages (e.g., Reheis and Redwine, 2008; Reheis et al., 2007, 2012). ~240 k.y.; individual pebble ages cluster into 4 groups with exposure ages of The timing and surface elevations of pluvial lake stages within the Mojave 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), 99.14 ± 12.07 ka (n = 4) and 239.9 ± River basin have been used to estimate the water balance in the Mojave River 52.16 ka (n = 1). These inferred large discharge events occurred during both watershed and the discharge required to sustain one or more pluvial lakes glacial and interglacial conditions. We demonstrate that bedload materials along the Mojave River course (Enzel and Wells, 1997). Although the source provide information about the frequency and duration of transport events in of the discharge necessary to produce and maintain pluvial lakes along the river systems. This approach could be further improved with additional mea- Mojave River course remains a topic of considerable debate (e.g., Enzel et al., surements of one or more cosmogenic nuclides coupled with models of river 2003, and references therein; Miller et al., 2010; Kirby et al., 2012; Lyle et al., discharge and pebble transport. 2012; Antinao and McDonald, 2013), estimates of the magnitude and timing of Mojave River discharge are based on spatially and temporally disparate allu vial, lacustrine, and marine sedimentary records. To our knowledge, no at- INTRODUCTION tempt has been made to test these hypotheses using a more direct measure of Mojave River paleodischarge. Arid landscapes shaped by ephemeral stream flow are challenging to char- We present 10 new cosmogenic 10Be exposure ages of individual quartzite acterize and model because of the complex spatial and temporal changes in pebbles collected from alluvium deposited on a strath terrace of the Mojave geomorphic processes and the incomplete records of climate factors that drive River near Barstow, California (Figs. 1 and 2). The age of alluvium on the strath those processes. Innovative approaches such as modeling the residence time is constrained by new optically stimulated (OSL) and infrared stimulated (IRSL) of sand in ephemeral streams based on luminescence ages (e.g., McGuire and luminescence ages. We hypothesize that pebble exposure ages record episodic Rhodes, 2015), using various types of tracers to track sand (e.g., Crickmore, large discharge events in the Mojave River. The geology and geomorphology 1967; Rathburn and Kennedy, 1978; Milan and Large, 2014) or coarser material of the Mojave River watershed, and constraints on these characteristics pro- For permission to copy, contact Copyright (e.g., Church and Hassan, 1992; Lamarre et al., 2005) are improving our ability vided by previous research in the region, provide a nearly ideal experimental Permissions, GSA, or [email protected]. to characterize these types of geomorphic systems, as are recent studies of setting to test this hypothesis. © 2015 Geological Society of America GEOSPHERE | Volume 11 | Number 4 Cyr et al. | Paleodischarge of the Mojave River Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1158/3334974/1158.pdf 1158 by guest on 23 September 2021 Research Paper 117.5°W 117.0°W 116.5°W 116.0°W 115.5°W 35.5° N NV CA N 35.5° AZ Si Area of Fig. 2A C So M co 35.0° N H a N 35.0° ca t Legend Elevation (m) Erosion rate (m/m.y.) N Mojave River dam 2595 [from Binnie et al., 2007] at the Forks 34.5° 52–170 290 171–280 Quartzite clast- km 281–660 bearing units 015 0203040 661–1600 1601–2700 117.5°W 116.5°W Figure 1. Hillshade of 10-m-resolution National Elevation Dataset (NED, http:// ned .usgs .gov/) of the region around the Mojave River basin, including the modern extent of the Mojave River water- shed (white line; U.S. Geological Survey Water Resources Maps and geographic information system data; http:// water .usgs .gov /maps .html), the modern course of the Mojave River and major tributaries (heavy blue lines), the Mojave River basin drainage network (gray lines), the spatial distribution of quartzite clast-bearing conglomerates (hachured areas; Dibblee, 1973; Morton and Miller, 2006), and the locations of catchment-averaged erosion rates reported by Binnie et al. (2007). Playas supported by Mojave River discharge are denoted by blue polygons (H—Harper Lake; C—Cronese Lakes; So—Soda Lake; Si—Silver Lake; M—Lake Manix; ca—Cady subbasin; co—Coyote subbasin; t—Troy subbasin; a—Afton subbasin). The southwestern edge of the Mojave River watershed, highlighted by the black-edged outline, denotes the approximate extent of the high-elevation, low-relief area of the San Bernardino Mountains referred to in the text. CA—California; NV—Nevada; AZ—Arizona. GEOSPHERE | Volume 11 | Number 4 Cyr et al. | Paleodischarge of the Mojave River Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1158/3334974/1158.pdf 1159 by guest on 23 September 2021 Research Paper 116.5625°W 116.5542°W 116.54583°W m 34.9583° N A B 0150 300450 600 Lake Manix deposits N 34.9583° Figure 2. (A) Lidar (light detection and ranging) elevation data set (1 m resolu- N tion) of the area near the sampling loca- tion, draped over a hillshade of the same Elevation (m) lidar data, and showing the extent of 34.9542° 631 strath terrace fragments where pebbles C were collected (white lines). Arrows in- dicate the view directions of the photos shown in shown in B and C, taken from the 116.5625°W B 116.5542°W 486 sampling location. (B) View to the south- west from the sample location showing the extent of the terrace surface. (C) View to the northwest from the sample loca- C D tion. Bluffs in the background and the ter- race edge in the middle-ground are Lake Manix lacustrine deposits into which the Lake Manix deposits strath is inset and that Mojave River allu- vium overlies. The modern channel of the Mojave River is at the base of the bluffs (dark vegetation line). (D) Close-up view of the sampled pavement. 1. The quartzite pebbles are sourced from metasedimentary and sedimen- nardino Mountains, which are ~5%–10% of the ~9500 km2 Mojave River drain- tary rocks exposed primarily in the upper elevations of the San Bernardino age area, receive >1000 mm/yr, whereas the remainder of the Mojave River Mountains (Fig. 1) (Dibblee, 1973; Sadler and Reeder, 1983; Morton and drainage area receives between 125 and 150 mm/yr (as measured near Baker, Miller, 2006). These rocks are dominantly conglomerates, and so the quartzite California; Enzel and Wells, 1997; Reheis et al., 2012). Hydrological records and pebbles that are eroded out of hillslopes are presorted and rounded, mini- historical accounts indicate that the discharge of the Mojave River is strongly mizing the possible effects on 10Be concentrations of grain comminution or correlated with precipitation in the San Bernardino Mountains; nearly the en- sorting during transport (e.g., Belmont et al., 2007; Carretier and Regard, tire mean annual discharge of ~9.5 × 106 m3 is derived from that 5%–10% of 2011; Carretier et al., 2015, and references therein).