Mojave River Research Articles

How old are the Mojave topographic dunes? The implications of new luminescence dating analyses from the Cady Mountains, Mojave Desert, southwest USA

The aeolian landforms of the Mojave Desert in the SW USA have been studied in detail over the last three decades, particularly in terms of their relationship to the region's topography and Pleistocene climate / sediment supply histories, as well as wider developments, such as the aeolian “sediment state” concept. In this context, the evolution of the Mojave River and its associated palaeolakes is thought to have been a key control on long-term sediment supply to aeolian systems, and luminescence dating chronologies for a range of (resulting) aeolian landforms have been related to these hydrological changes. Here we argue that at least some of these aeolian chronologies need to be re-assessed.We focus on luminescence chronologies for aeolian landforms within and marginal to the Cady Mountains, a mountain block adjacent to the Mojave River and palaeolake Manix, east of Barstow, California. We demonstrate that quartz in this locale exhibits several malign luminescence properties, and that low temperature K-feldspar infrared stimulated luminescence (IRSL) consistently exhibits high anomalous fading rates. Both contribute to age underestimation. We address these issues via post-infrared IRSL (pIRIR) and post-isothermal post-IR (pIt-IR) analyses of K-feldspars. The resulting ages span the last ~120 ka and imply phases of aeolian activity of a substantially greater antiquity than previously inferred. Notably, at one well-studied site – the Soldier Mountain sand ramp – the new ages suggest a landform dating not to Marine Isotope Stage (MIS) 1 or 2, as previously suggested, but more likely to MIS 5. The Cady Mountain record indicates that the only period of the last glacial cycle lacking evidence for aeolian sedimentation is ~40–9 ka, broadly consistent with expectations of increased regional humidity. These results also suggest that site topographic context may influence the age structure of aeolian deposits. In this instance, sand ramps consistently represent the oldest type of deposit (range ~ 40–120 ka), while early-mid Holocene dune accumulation is associated with sandsheets and valley-fill sands. Based on these findings, we argue that there is a need to critically re-assess the existing regional luminescence age database, and that there is potential to significantly revise our understanding of the region's aeolian system responses, and associated paleoenvironmental interpretations.

Injuries and Abnormalities of the Southwestern Pond Turtle (Actinemys pallida) in the Mojave River of California

La tortuga de estanque del suroeste (Actinemys pallida) es una tortuga semiacuática que, de vez en cuando, pasa tiempo en la tierra para asolearse, ovipositar, realizar movimientos intermitentes e hibernar en tierra. El uso de ambos medios, acuático y terrestre, expone a las tortugas semiacuáticas a un mayor riesgo de lesiones o mortalidad por inundaciones, intentos de depredación y otros peligros ambientales (por ejemplo, actividades humanas, tales como choques vehiculares, etc.). Recolectamos datos de lesiones y anormalidades morfológicas de tortugas adultas en tres sitios de estudio a lo largo del río Mojave en el condado de San Bernardino, California. Un sitio en la sección superior del río Mojave (en adelante conocido como UHMRS) y dos sitios en la sección inferior del río Mojave (en adelante conocidos como LHMRS). Los estudios se realizaron durante la temporada de mayor actividad de las tortugas entre mayo y octubre de 1998–1999 y nuevamente entre abril y septiembre de 2016–2019. Se capturó un total de 84 A. pallida entre todos los sitios y todos los años. El 17% (n = 8) de las tortugas capturadas en UHMRS presentaban anomalías en el caparazón (variaciones naturales en la morfología del caparazón o de los huesos). Se encontraron lesiones (daños infligidos por la fuerza al caparazón o al cuerpo) en el 68% (n = 26) de las tortugas capturadas en los dos sitios de LHMRS combinados y en el 78% (n = 36) de las tortugas capturadas sólo en el UHMRS. El 74% (n = 62) de las tortugas tenían lesiones en todos los sitios combinados. No hubo diferencias estadísticas en la proporción de tortugas heridas y no heridas entre los sexos para los dos sitios LHMRS combinados o el UHMRS. La longitud promedio del caparazón no fue significativamente diferente entre las tortugas heridas y las no heridas en estos mismos lugares. La mayoría de las tortugas capturadas en todos los sitios presentaban lesiones, lo que podría ser un indicador del alcance de las amenazas a las que se enfrentan estas tortugas.

Geomorphic history of Lake Manix, Mojave Desert, California: Evolution of a complex terminal lake basin

Pluvial lake deposits in the Basin and Range province have long been exploited for records of hydrologic and climatic change. To obtain the most accurate reconstructions, a thorough understanding of the geomorphic evolution of such lake basins must be grounded in field stratigraphy and mapping, which are often lacking or are not well integrated with core studies. We conducted extensive stratigraphic investigation, mapping, and dating of exposed strata in the Lake Manix basin, former terminus of the Mojave River, California, during the Pleistocene to provide such a background and to supplement interpretations from a 45-m-long core.In this paper, we emphasize the geomorphic and tectonic processes that shaped the evolution of the Manix basin after the arrival of the Mojave River in the early-middle Pleistocene. The main processes include (1) sedimentary infilling of the lake basin, (2) interaction among the subbasins as controlled by internal sills and sill failures, (3) tectonics, and (4) climate change. From about 500 to 190 ka, Lake Manix was confined to its western subbasins, and fluctuated in response to climate change and perhaps to brief diversions of the river into the upstream Harper Lake basin. During this time, about 24 m of sediment accumulated near the confluence of the Mojave River and Manix Wash, and a 15-km-long clastic wedge built into the western basins, gradually filling the accommodation space.Two basin-integration events subsequently occurred, both caused by failure of sills that lay on sheared deposits along the Manix fault zone. When Lake Manix rose at the onset of glacial marine isotope stage 6, the sill on the south flank of Buwalda Ridge failed and triggered a catastrophic flood that entered the new, lower elevation, previously endorheic Afton subbasin. Subsequent lake-level changes affected both subbasins, but the thickest deposits after the flood are preserved in the Afton subbasin. The area upstream of Buwalda Ridge was only submerged during relatively high lake levels. Thus, sediment records interpreted from the core site and nearby outcrops in part reflect a geomorphic event (basin integration) rather than climatic conditions. The sill of Lake Manix after the integration event lay at the northeast end of the Afton subbasin, athwart strands of the Manix fault zone. At about 25 ka, this eastern sill was rapidly incised, and the river advanced to terminate in Lake Mojave in the Soda and Silver Lake basins. Recognition and dating of integration events, common for lake-river interactions in active tectonic settings, provide important context for interpreting paleoclimate records from sediments and provide temporal and spatial constraints on the biogeography of aquatic species.

Applying Biodiversity Metrics as Surrogates to a Habitat Conservation Plan

Unabated urbanization has led to environmental degradation and subsequent biodiversity loss across the globe. As an outcome of unmitigated land use, multi-jurisdictional agencies have developed land use plans that attempt to protect threatened or endangered species across selected areas by which some trade-offs between harm to species and additional conservation approaches are allowed among the partnering organizations. Typical conservation plans can be created to focus on single or multiple species, and although they may protect a species or groups of species, they may not account for biodiversity or its protection across the given area. We applied an approach that clustered deductive habitat models for terrestrial vertebrates into metrics that serve as surrogates for biodiversity and relate to ecosystem services. In order to evaluate this process, we collaborated with the partnering agencies who are creating a Multi-Species Habitat Conservation Plan in southern California and compared it to the entire Mojave Desert Ecoregion. We focused on total terrestrial vertebrate species richness and taxon groupings representing amphibians, birds, mammals, and reptiles, and two special status species using the Normalized Index of Biodiversity (NIB). The conservation planning area had a lower NIB and was less species rich than the Mojave Desert Ecoregion, but the Mojave River riparian corridor had a higher NIB and was more species-rich, and while taxon analysis varied across the geographies, this pattern generally held. Additionally, we analyzed desert tortoise (Gopherus agassizii) and desert kit fox (Vulpes macrotis arsipus) as umbrella species and determined that both species are associated with increased NIB and large numbers of species for the conservation area. Our process provided the ability to incorporate value-added surrogate information into a formal land use planning process and used a metric, NIB, which allowed comparison of the various planning areas and geographic units. Although this process has been applied to Apple Valley, CA, and other geographies within the U.S., the approach has practical application for other global biodiversity initiatives.

Western pond turtles in the Mojave Desert? A review of their past, present, and possible future

The western pond turtle (WPT) was formerly considered a single species (ActinemysorEmys marmorata) that ranged from southern British Columbia, Canada to Baja California, México. More recently it was divided into a northern and a southern species. WPTs are found primarily in streams that drain into the Pacific Ocean, although scattered populations exist in endorheic drainages of the Great Basin and Mojave deserts. Populations in the Mojave Desert were long thought to be restricted to the Mojave River, but recently another population was documented in Piute Ponds, a terminal wetland complex associated with Amargosa Creek on Edwards Air Force Base. WPT fossils in the Mojave Desert are known from the Miocene to the Pleistocene. Recently, Pleistocene fossils have been found as far into the desert as Salt Springs, just south of Death Valley. The oldest fossil records suggest that WPTs were present in wetlands and drainages of the geological feature known as the Mojave block prior to the uplift of the Sierra Nevada Range about 8 Ma and prior to the ~ 3 Ma uplift of the Transverse Ranges. Archaeological records document use of turtles by Native Americans for food and cultural purposes 1,000 or more years ago at the Cronese Lakes on the lower Mojave River and Oro Grande on the upper river. The first modern publication documenting their presence in the Mojave River was 1861. Museum specimens were collected as early as 1937. These fossil and early literature records support the indigenous status of WPTs to the Mojave River. However, mtDNA-based genetic evidence shows that Mojave River turtles share an identical haplotype with turtles on the California coast. Limited nuclear data show some minor differences. Overdraft of water from the Mojave River for municipal and agricultural uses, urban development, and saltcedar expansion are threats to the continued survival of WPTs in the Mojave River.

Effectiveness of Stormwater Best Management Practices in Headwater Streams to Mitigate Harmful Algal Blooms

Pollution inputs in surface waters have resulted in extensive impairments to water resources; however, the effectiveness of stormwater best management practices (BMPs) in reducing pollution inputs related to harmful algal blooms (HABs) in headwater streams has not been widely reported. Skypark, Santa’s Village, is an outdoor recreation area in the semiarid San Bernardino National Forest, California. Recreational activities and impervious surfaces at the site contribute pollution to Hooks Creek, a first-order headwater tributary of the Mojave River. The Natural Resources Conservation Service designed and constructed a stormwater sediment erosion control basin system to reduce site gully erosion and improve surface water quality in situ and downstream. Basin water quality was tested biweekly for parameters associated with HABs including temperature, dissolved oxygen, pH, turbidity, conductivity, nitrate (NO3−), and ammonium (NH4+) in situ during wet and dry seasons, with periodic testing for total suspended solids (TSS), total dissolved solids (TDS), total coliform (TC), and Escherichia coli (EC). The BMP structure was effective in lowering temperature and pH and reducing NO3−, TDS, and turbidity during precipitation events, and increased pH levels and lower concentrations of TSS, TC, and EC were present during the dry season. Despite these advantages, the BMP was ineffective in reducing (NH4+) concentrations, a primary contributor to HABs, with 100% of the samples exceeding regulatory criteria throughout the study period. Results highlight the benefits and limitations of stormwater BMPs in protecting water resources from downstream HABs to ensure water resources are protected for current and future generations.

The role of eolian-fluvial interactions and dune dams in landscape change, late Pleistocene–Holocene, Mojave Desert, USA

AbstractThe formation of the Kelso Dunes in the eastern Mojave Desert, California, was a landscape-changing event triggered by an increase in sediment supply that followed the incision of Afton Canyon by the Mojave River ca. 25 ka. Eastward migration of sand dunes occurred along a well-defined eolian transport corridor. Dunes temporarily blocked washes resulting in substantial aggradation of eolian and fluvial sediments. Stratigraphic exposures reveal numerous fining-up sequences with interbedded eolian sands that provide evidence of dune dams and subsequent aggradation. Luminescence ages reveal that dune blocking and aggradation correspond to a regional pulse of alluvial fan sedimentation that occurred ca. 14–9 ka. Meanwhile, relative landscape stability occurred downstream of dune dams, resulting in the formation of a moderately developed soil on abandoned fluvial deposits. The next pulse of alluvial fan activity ca. 6–3 ka likely resulted in the breaching of the dune dams, followed by incision. Eolian system sediment state theory suggests that eolian activity in the Mojave Desert is closely tied to enhanced sediment supply, primarily related to the Mojave River–Lake Mojave system. Our data suggests that Intermittent Lake Mojave I, ca. 26–22 ka, triggered a large dune-building event that impounded massive amounts of sediment derived from alluvial fans deposited during the Pleistocene-Holocene transition. Breaching of dune dams and sediment recycling may have also increased sediment supply that contributed to late Holocene eolian activity. This profound impact on the regional geomorphology highlights the critical importance of eolian-fluvial interactions in desert environments.

Channel mobility drives a diverse stratigraphic architecture in the dryland Mojave River (California, USA)

AbstractThe links between flood frequency and rates of channel migration are poorly defined in the ephemeral rivers typical of arid regions. Exploring these links in desert fluvial landscapes would augment our understanding of watershed biogeochemistry and river morphogenesis on early Earth (i.e. prior to the greening of landmasses). Accordingly, we analyse the Mojave River (California), one of the largest watercourses in the Great Basin of the western United States. We integrate discharge records with channel‐migration rates derived from dynamic time‐warping analysis and chronologically calibrated subsidence rates, thereby constraining the river's formative conditions. Our results reveal a slight downstream decrease in bankfull discharge on the Mojave River, rather than the downstream increase typically exhibited by perennial streams. Yet, the number of days per year during which the channel experiences bankfull or higher stages is roughly maintained along the river's length. Analysis of historical peak flood records suggests that the incidence of channel‐formative events responds to modulation in watershed runoff due to the precipitation in the river's headwaters over decades to centuries. Our integrated analysis finally suggests that, while maintaining hydraulic geometries that are fully comparable with many other rivers worldwide, ephemeral desert rivers akin to the Mojave are capable of generating a surprisingly wide range of depositional geometries in the stratigraphic record. © 2020 John Wiley & Sons, Ltd.

Selected trace-elements in alluvium and rocks, western Mojave Desert, southern California

Concentrations of twenty-seven elements, including naturally-occurring water-quality contaminants arsenic, chromium, and uranium, were measured in 217 samples of alluvium and rock from the western Mojave Desert, southern California, using portable (pXRF) and laboratory (LXRF) X-ray fluorescence. Comparison of measurements with NIST-traceable standards was good, although pXRF overestimated iron compared to LXRF. Results suggest pXRF survey data are sufficiently accurate to assess regional geochemical differences in geologic-source terrains. Principal component analysis showed rubidium and potassium were associated with alluvium eroded from felsic terrain, while iron, copper, chromium, and to a lesser extent titanium, manganese, and nickel were associated with alluvium eroded from mafic terrain. Zinc, vanadium, and arsenic were associated with alluvium eroded from hydrothermal terrain. Elemental assemblages associated with different source terrains were traced spatially to identify the source and composition of alluvium composing aquifers pumped for water supply. Changes in geologic source terrain to the Mojave River, associated with movement along the San Andreas Fault over the past one to five million years, reduced the mafic fraction and increased the felsic fraction of alluvium deposited to the regionally important floodplain aquifer along the Mojave River—lowering chromium concentrations in alluvium through geologic time. Comparison of pXRF and sequential extraction data from 40 samples showed arsenic and uranium were more abundant on the surfaces of mineral grains, while chromium and vanadium remained mostly within unweathered mineral grains—suggesting arsenic and uranium may be more readily mobilized into groundwater with changes in pH, redox, or ionic strength than chromium or vanadium.

Unseen Dust Emission and Global Dust Abundance: Documenting Dust Emission from the Mojave Desert (USA) by Daily Remote Camera Imagery and Wind‐Erosion Measurements

AbstractA large gap in understanding the effects of atmospheric dust at all spatial scales is uncertainty about how much and whence dust is emitted annually. Direct digital recording of dust emission at high spatial and temporal resolution would together with periodic flux measurements support improved estimates of local‐scale dust flux. Remote camera recording of dust‐emitting settings on and around Soda Lake (Mojave Desert) was conducted every 15 min during daylight between 10 November 2010 and 31 December 2016. Examination of 135,000 images revealed frequent dust events characterized as dust days, 68 per year on average, when dust plumes obscured mountains beyond areas of dust generation. Dust was not observed from examination of satellite retrievals (Moderate Resolution Imaging Spectroradiometer and Geostationary Operational Environmental Satellite System) during six cloudless days of extreme emission. Estimated masses of emitted dust fractions were made from measured horizontal mass flux, sediment particle sizes, and areas of dust generation. Between April 2000 and June 2013, nearly 4 Tg/year of dust (particles < 63 μm) were emitted across the study area. Higher rates (about 7 Tg/year) were estimated following the December 2010 Mojave River flood that deposited sediment across the lake basin. Within the Mojave and Great Basin deserts of western North America, many settings akin to those at Soda Lake similarly emit dust that is rarely detected in satellite retrievals, implying that local and regional dust emissions from western North America are far underestimated and that, by extension to relatively small dust‐source areas across all drylands, global dust emissions may also be greatly underestimated.

Resignation, and: Reckoning

Resignation, and: Reckoning Jane Miller (bio) RESIGNATION This world is not puréed, but for youI make it so. Every day a blender grindsraw morning smooth against the chanceyou will disgorge food barely chewed. When I was young you forced the spoon,such rough devotion to nourish, to schoolthe stubborn in me. Now I motherproduce to appease. The texture your life wants, I cannot tender,though I wish it could be. Dementiahas choked you out of speech,your memory, a raging fist.What kindness can, let me blendwith anger, carrot, banana, fear. [End Page 119] RECKONING The coffin where I keep my dead wordsis open. The jewelry box, pink like young skin,where I store trinkets is shut. The ballerina inside her pink tutu face downon her footed spring, waits for the handthat opens her. She must be tired, always rising over the horizon of my face. Outside, an egret standsin the marsh, a white flag marking my surrenderto the wind. On my walls, swallows painted in their frames are gone. Where wormswill crawl, they peck at my casket words, beakstyping on pine, fresh-cut unlined. In bed, I am a ghost form breathing rufflesinto sheets pulled taut as paper,my wallpaper wilting like corsages that once romanced my wrist, my shoulderwhere babies clung. On the blanket stand,my burial dress, all scattered florals. I will be nosegay, cabbage headwith blue hydrangea perm,my eyes’ fringed curtains finally [End Page 120] closed, my nose never moreto be unstuffed, and my mouth,with helpful stitching, seamed shut. Where are my babies now? I have seen enough bodiesto know how rouge helps deathblush. I hope the satin words comfort me,that the birds leave enough. [End Page 121] Jane Miller Jane Miller’s poetry has appeared in the Iron Horse Literary Review, Summerset Review, cahoodaloodaling, Mojave River Review, and Pittsburgh Poetry Review, among others. A nominee for Best New Poets and Best of the Net, she was a finalist in the 2017 Red Wheelbarrow Poetry Contest. Copyright © 2018 Berea College

A complex origin for the Kelso Dunes, Mojave National Preserve, California, USA: A case study using a simple geochemical method with global applications

The Kelso Dune field in southern California is intriguing because although it is of limited areal extent (~100km2), it has a wide variety of dune forms and contains many active dunes (~40km2), which is unusual in the Mojave Desert. Studies over the past eight decades have concluded that the dunes are derived primarily from a single source, Mojave River alluvium, under a dominant, westerly-to-northwesterly wind regime. The majority of these studies did not, however, present data to support the Mojave River as the only source. We conducted mineralogical and geochemical studies of most of the 14 geomorphically defined dune groups of the Kelso Dune field as well as potential sand sources, alluvial sediments from the surrounding mountain ranges. Results indicate that sands in the nine western dune groups have K/Rb and K/Ba (primarily from K-feldspar) compositions that are indistinguishable from Mojave River alluvium (westerly/northwesterly winds) and Budweiser Wash alluvium (southwesterly winds), permitting an interpretation of two sources. In contrast, sands from the five eastern dune groups have K/Rb and K/Ba values that indicate significant inputs from alluvial fan deposits of the Providence Mountains. This requires either rare winds from the east or southeast or, more likely, aeolian reworking of distal Providence Mountain fan sediments by winds from the west, at a rate greater than input from the Mojave River or other western sources. The results indicate that even a small dune field can have a complex origin, either from seasonally varying winds or complex alluvial-fan-dune interaction. Application of K/Rb and K/Ba in K-feldspar as a provenance indicator could be used in many of the world's ergs or sand seas, where dune origins are still not well understood or are controversial. Four examples are given from Africa and the Middle East where such an approach could yield useful new information about dune sand provenance.

Evidence for insolation and Pacific forcing of late glacial through Holocene climate in the Central Mojave Desert (Silver Lake, CA)

Silver Lake is the modern terminal playa of the Mojave River in southern California (USA). As a result, it is well located to record both influences from the winter precipitation dominated San Bernardino Mountains – the source of the Mojave River – and from the late summer to early fall North American monsoon at Silver Lake. Here, we present various physical, chemical and biological data from a new radiocarbon-dated, 8.2m sediment core taken from Silver Lake that spans modern through 14.8calkaBP. Texturally, the core varies between sandy clay, clayey sand, and sand-silt-clay, often with abrupt sedimentological transitions. These grain-size changes are used to divide the core into six lake status intervals over the past 14.8calkaBP. Notable intervals include a dry Younger Dryas chronozone, a wet early Holocene terminating 7.8 – 7.4calkaBP, a distinct mid-Holocene arid interval, and a late Holocene return to ephemeral lake conditions. A comparison to potential climatic forcings implicates a combination of changing summer – winter insolation and tropical and N Pacific sea-surface temperature dynamics as the primary drivers of Holocene climate in the central Mojave Desert.

Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of10Be

The paleohydrology of ephemeral stream systems is an important constraint on paleoclimatic conditions in arid environments, but remains difficult to measure quantitatively. For example, sedimentary records of the size and extent of pluvial lakes in the Mojave Desert (southwestern USA) have been used as a proxy for Quaternary climate variability. Although the delivery mechanisms of this additional water are still being debated, it is generally agreed that the discharge of the Mojave River, which supplied water for several Pleistocene pluvial lakes along its course, must have been significantly greater during lake highstands. We used the 10 Be concentrations of 10 individual quartzite pebbles sourced from the San Bernardino Mountains and collected from a ca. 25 ka strath terrace of the Mojave River near Barstow, California, to test whether pebble ages record the timing of large paleodischarge of the Mojave River. Our exposure ages indicate that periods of discharge large enough to transport pebble-sized sediment occurred at least 4 times over the past ∼240 k.y.; individual pebble ages cluster into 4 groups with exposure ages of 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), 99.14 ± 12.07 ka (n = 4) and 239.9 ± 52.16 ka (n = 1). These inferred large discharge events occurred during both glacial and interglacial conditions. We demonstrate that bedload materials provide information about the frequency and duration of transport events in river systems. This approach could be further improved with additional measurements of one or more cosmogenic nuclides coupled with models of river discharge and pebble transport.

Downstream MET-IRSL single-grain distributions in the Mojave River, southern California: Testing assumptions of a virtual velocity model

We use samples from a prior study (McGuire and Rhodes, 2015) to investigate the bleaching trend of Mojave River sand in more detail. We present new single grain data which provides insight into how previously presented multiple-grain luminescence signals decrease downriver. The single grain dose distributions allow for a test of the assumption that multiple-grain equivalent dose (De) is representative of fluvial transport down the Mojave River. For samples at the Forks, Victorville and Barstow, with laboratory luminescence sample codes J0262, J0267 and J0265, respectively, inspection of the kernel density estimate (KDE)-generated probability density function supports the assumption, though is does not prove it directly. Implementing a Kolmogorov–Smirnov (K–S) test shows that the downriver samples are statistically different from each other, suggesting bleaching as the primary mechanism for changes is dose distribution downriver. Single-grain dose distributions show that the De of the Afton Canyon sample (J0260) is not representative of grain travel from source to sink, but instead likely a result of local mixing of sediment populations. This result is confirmed by visual inspection of the KDE, and quantitatively using the K–S test. As has been noted by several authors, the single-grain dose distribution in active channels of rivers may represent a worst case in terms of poor bleaching, due to mixing of older-age populations (Cunningham et al., 2014; Jain et al., 2004). This observation holds for our data set and presents an opportunity to test and develop luminescence techniques to determine provenance. In particular, future sampling in the vicinity of the Afton Canyon site has the potential to identify the entry point of a poorly-bleached population.

Geophysical, remote sensing, GIS, and isotopic applications for a better understanding of the structural controls on groundwater flow in the Mojave Desert, California

Abstract Study region Mojave Desert, USA. Study focus An integrated (near-surface geophysics, remote sensing, isotopic analyses) study was conducted in the Mojave River Basin and Morongo Groundwater Basin to investigate potential effects that the Helendale Fault [HF] and basement uplifts might have on groundwater flow in the Mojave Desert. New hydrological insights for the region The HF traces were mapped using LiDAR and Geoeye-1 imagery (surface) and magnetic profiles (subsurface). Shallow basement parallel to and west of the HF was detected using the Vertical Electrical Soundings (VESs). Conductive water-saturated breccia was detected along the HF using the Very Low Frequency (VLF) electromagnetic measurements. Isotopic analyses (δD and δ 18 O) for groundwater samples from productive shallow wells, and springs sampled west of the HF and the basement uplift are less depleted (Group I: Fifteenmile Valley Groundwater sub-basin [FVGS]; average δD: −86.8‰; δ 18 O: −11.8‰) than samples east of the basement uplift (Group II: Lucerne Valley Groundwater sub-basin [LVGS]; average δD: −95.0‰; δ 18 O: −12.1‰), whereas samples proximal to, the fault have compositions similar to Group I but show evidence for mixing with Group II compositions (Group III; average δD: −88.8‰; δ 18 O: −11.5‰). Findings are consistent with the HF channeling groundwater from the San Bernardino Mountains with basement uplifts acting as barriers to lateral groundwater flow and could be applicable to similar settings across the Mojave Desert and elsewhere worldwide.

Simulating Arsenic Mitigation Strategies in a Production Well

Water from well 5N/4W-31A1 in the regional aquifer in the Mojave River groundwater basin 97 kilometers north east of Los Angeles, California, occasionally exceeds the U.S. EPA Maximum Contaminant Limit for arsenic of 10 micrograms per liter (μg/L). Coupled well-bore flow and depth-dependent water-quality sampling for this well show arsenic concentrations less than 0.12 μg/L entering the well from the aquifer in the upper 163 meters (m) below land surface (bls). Arsenic concentration increase with depth to a maximum of 17.6 μg/L at 213 m bls. High arsenic in the deeper part of the well are associated with pH greater than 9 and dissolved oxygen concentrations less than 0.2 milligrams per liter. An axially-symmetric, radial groundwater flow simulation, developed using the computer program AnalyzeHOLE, was used to simulate flow to the well under pumping conditions. Simulations show that modifying the existing well design by eliminating the two deepest screened intervals below 189 m bls would reduce arsenic concentrations in the surface discharge of the well about 25 percent with a 30 percent reduction in yield. Such well modification may reduce or eliminate the need for costly arsenic treatment or blending of waters from different sources to reduce arsenic concentrations in water delivered to consumers.

Determining fluvial sediment virtual velocity on the Mojave River using K-feldspar IRSL: Initial assessment

The Mojave River of Southern California was chosen as a field site to investigate the applicability of luminescence dating to sediment transport rate problems. Grains in the active channel of the river are expected to show signs of partial bleaching and this makes it difficult to determine time since deposition accurately. A modification of the multiple elevated temperature post-IR IRSL (METpIRIR) procedure, (Buylaert et al., 2009; Li and Li, 2011), was used for K-feldspar grains (175–200 μm) at temperature increments of 50, 95, 140, 185, and 230 °C in order to provide more information about relative signal bleaching among samples. The measurements show an exponential decrease in equivalent dose (De) with distance down the Mojave River. Higher temperature pIRIR signals are bleached more slowly than lower temperature ones (Buylaert et al., 2009). The De for samples at 50 °C is roughly constant along the river. These results suggest cyclical bleaching and burial as grains are transported downriver and higher energy (deeper) traps are vacated. The pattern of De values for the Mojave River can be used to constrain the sediment transport rate for this river by building a model of growth and bleach for each temperature increment. A bleaching experiment was run with multiple aliquot samples for direct sunlight exposure times of 0, 10, 30, 300, 1000, 3000, 10,000, and 30,000 s. The MET-pIRIR procedure was applied at each temperature increment for each exposure time aliquot and the results for all exposure times were fit to the general order kinetics equation using a non-linear regression. The bleaching parameters were used in conjunction with the SAR growth curves to build a model of partial bleaching of grains during transport that is fitted with a χ2 test to the pIRIR data from the Mojave River. This model is not a unique solution, but can be used to assess the likelihood of various sediment transport regimes.

Geochronology and paleoenvironment of pluvial Harper Lake, Mojave Desert, California, USA

Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500ka. Previous studies inferred a polar jet stream south of 35°N at 18ka and at ~40°N at 17–14ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to >30,00014CyrBP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~45–40ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40ka, but shifted to 40°N by ~35ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~45–40ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.

Genetic characterization and management of the endangered Mohave tui chub

The Mohave tui chub (Siphateles bicolor mohavensis) is the only fish native to the Mojave River, California. The fish were displaced by introduced arroyo chubs (Gila orcutti) throughout most of their range, starting in the 1930s. Two potentially relictual populations and two transplanted populations were genetically characterized using 12 microsatellite DNA loci, along with contemporary cyprinid populations in the Mojave River. We found only un-hybridized Mohave tui chubs in the refuge populations, and only un-hybridized arroyo chubs in the Mojave River. The two largest Mohave tui chub populations (Lake Tuendae and China Lake) exhibit similar, comparatively high genetic variation. Another large population (Camp Cady) with low genetic diversity shows the effect of a bottleneck of ten individuals during the historic founding event. The fourth population (MC Spring) has the fewest alleles, lowest heterozygosity, and is the most divergent, suggesting that genetic drift from a persistently low effective population size has reduced genetic diversity since its apparent isolation in 1934. We recommend instituting artificial gene flow to rebuild genetic variation in Camp Cady from both Lake Tuendae and China Lake, and the establishment of new populations with founders from both Lake Tuendae and China Lake. Additionally, we comment on the infeasibility of restoring populations of Mohave tui chub in their historic habitats.