Mojave River Research Articles

A half-million-year record of paleoclimate from the Lake Manix Core, Mojave Desert, California

Pluvial lakes in the southwestern U.S. responded sensitively to past climate through effects on rainfall, runoff, and evaporation. Although most studies agree that pluvial lakes in the southwestern U.S. reached their highest levels coeval with glacial stages, the specific timing of increased effective moisture and lake-level rise is debated, particularly for the southwesternmost lakes. We obtained a 45-m core of lacustrine sediment from Lake Manix, the former terminus of the Mojave River prior to about 25ka, and supplemented data from the core with outcrop studies. These sediments provide a robust record of Mojave River discharge over the last half-million years. Lake Manix persisted from OIS 12 through early OIS 2, including during interstadial OIS 3 and interglacials OIS 5, 7, and 9. The ostracode faunal record displays a shift from an unexpectedly warm, summer-dominated lake hydrology during OIS 12 to predominantly colder, winter-dominated conditions afterwards. The ostracode-based stable isotope record displays a large degree of intra-sample variability and does not mimic other well-known isotopic records of climate change. Evaporation likely buffered the Manix δ18O record from most of the expected isotopic differences between interglacial and glacial-interval discharge. Isotopically depleted and stable lakes occurred only four to six times, most notably during OIS 7 and OIS 9. Internal drainage-basin changes also affected the isotopic record. Persistence of lakes in the Manix basin during interglacials requires atmospheric or oceanic circulation controls on the mean position of the Pacific storm track other than large ice sheets. We propose that the relative strength and sign of the Northern Annular Mode (NAM) and its influence on atmospheric river-derived precipitation is a potential explanation.

A 9170-year record of decadal-to-multi-centennial scale pluvial episodes from the coastal Southwest United States: a role for atmospheric rivers?

A well-dated, 9170 calendar year before present (cal yr BP) paleohydrologic reconstruction is presented from Lower Bear Lake in the San Bernardino Mountains of the coastal southwest United States. This new multi-proxy record is characterized by alternating organic-rich/carbonate-rich sediment units, interpreted to reflect hydrologically-forced changes in the lake's depositional environment. Our interpretation of the proxy data indicates nine decadal-to-multi-centennial pluvial episodes (PE) over the past 9170 cal yr BP. Of these nine inferred pluvials, five are interpreted as more pronounced based on their combined proxy interpretations: (PE-V) 9170?–8250, (PE-IV) 7000–6400, (PE-III) 3350–3000, (PE-II) 850–700, and (PE-I) 500–476 (top of core) cal yr BP. The Lower Bear Lake record indicates that the San Bernardino Mountains, source region for the Mojave River and its terminal playa, was wet during the same periods (within dating errors), to several of the major pluvials proposed from the lakes in the sink of the Mojave River. Our comparison extends north also to Tulare Lake, which drains the southcentral-western Sierra Nevada Mountains. This temporally and spatially coherent signal indicates that a similar climate forcing acted to increase regional wetness at various times during the past 9170 cal yr BP. As originally proposed by Enzel, Ely, and colleagues (e.g., Enzel et al., 1989; Enzel, 1992; Ely et al., 1994; Enzel and Wells, 1997), we too contend that Holocene pluvial episodes are associated with changing the frequency of large winter storms that track across a broad region at decadal-to-multi-centennial timescales. We build upon their hypothesis through the addition of new and better-dated site comparisons, recent advances in the understanding of atmospheric rivers, and improved knowledge of the ocean–atmosphere dynamics that caused the early 20th century western United States pluvial.

Asian Fish Tapeworm Bothriocephalus acheilognathi in the Desert Southwestern United States

The Asian fish tapeworm Bothriocephalus acheilognathi (Cestoda: Bothriocephalidea) is an introduced fish parasite in the southwestern United States and is often considered a serious threat to native desert fishes. Determining the geographic distribution of nonnative fish parasites is important for recovery efforts of native fishes. We examined 1,140 individuals belonging to nine fish species from southwestern U.S. streams and springs between January 2005 and April 2007. The Asian fish tapeworm was present in the Gila River, Salt River, Verde River, San Pedro River, Aravaipa Creek, and Fossil Creek, Arizona, and in Lake Tuendae at Zzyzx Springs and Afton Canyon of the Mojave River, California. Overall prevalence of the Asian fish tapeworm in Arizona fish populations was 19% (range = 0-100%) and varied by location, time, and fish species. In California, the prevalence, abundance, and intensity of the Asian fish tapeworm in Mohave tui chub Gila bicolor mohavensis were higher during warmer months than during cooler months. Three new definitive host species--Yaqui chub G. purpurea, headwater chub G. nigra, and longfin dace agosia chrysogaster--were identified. Widespread occurrence of the Asian fish tapeworm in southwestern U.S. waters suggests that the lack of detection in other systems where nonnative fishes occur is due to a lack of effort as opposed to true absence of the parasite. To limit further spread of diseases to small, isolated systems, we recommend treatment for both endo- and exoparasites when management actions include translocation of fishes.

Phylogeography of the ground squirrel subgenusXerospermophilusand assembly of the Mojave Desert biota

AbstractAim The Mohave ground squirrel (Xerospermophilus mohavensis) is one of a few endemic species of the Mojave Desert of south‐western North America. We describe phylogeographic patterns within this species and its sister taxon (Xerospermophilus tereticaudus) and test hypotheses concerning their biogeographical history using genetic signatures of stable versus expanding populations. We compare these patterns with those of other Mojave species to evaluate the role of vicariance in producing phylogeographic structure during the assembly of the Mojave Desert biota.Location The Mojave Desert and adjacent desert regions of south‐western North America.Methods Complete cytochromebgene sequences ofX. mohavensis(46 individuals representing 11 localities) andX. tereticaudus(38 individuals representing 14 localities) were analysed using Bayesian methods to infer phylogenetic relationships. Genetic signals of stable or expanding populations were examined based on the distribution of recent mutations and pairwise differences, as well as with a coalescent‐based approach.Results The two species are reciprocally monophyletic and may have diverged in response to the late Pliocene–early Pleistocene uplift of the Transverse Ranges and Mojave block. Little phylogeographic structure is evident withinX. mohavensis, but there is a signature of northern expansion from a presumably full‐pluvial refugium in the Mojave River basin. Four geographic subgroups are evident withinX. tereticaudus, and there is a signature of northern expansion from a presumably full‐pluvial refugium in the Sonoran coastal plains. Roughly congruent phylogeographic patterns are found within five arid‐adapted taxa, indicating a strong element of vicariance during the assembly of the generally transitional Mojave Desert biota.Main conclusions We present a preliminary model for the historical assembly of the Mojave Desert biota that indicates a strong vicariant element producing autochthonous lineages (includingX. mohavensis) that diverged during the major geological and climatic events of the last 5 Myr. Phylogeographic partitioning within the Mojave Desert underscores the necessity of immediate conservation measures for this unique and fragile arid ecosystem that is locked between two large metropolitan population centres and is the target of continued adverse environmental impact.

Evidence for crustal degassing of CF 4 and SF 6 in Mojave Desert groundwaters

Dissolved tetrafluoromethane (CF 4) and sulfur hexafluoride (SF 6) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF 4 and SF 6 are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF 4 from the lithosphere. A gradual basin-wide enhancement in dissolved CF 4 and SF 6 concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF 4 and SF 6 concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF 4, but not of SF 6, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF 4 and SF 6 into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF 4 and SF 6.

A History of Gila Monster (Heloderma suspectum cinctum) Records from California with Comments on Factors Affecting their Distribution

The Gila monster (Heloderma suspectum), widely distributed in parts of the Mojave, Sonoran and Chihuahuan Deserts of the southwestern United States and northwestern Mexico, is rare in California. However, during the last 153 years, as many as 26 credible records have been documented from four California counties. Habitat in which the species has been observed in California is characterized by rocky, deeply incised topography, in most cases, associated with large and relatively high mountain ranges. Most localities are associated with riparian areas (including the lower Colorado River) and range from near sea level to over 1,200 m. All records except one (Mojave River) occur east of about 116° longitude. Records documented with photographs or museum specimens generally show color patterns diagnostic of the geographically expected subspecies H. s. cinctum. The distribution of the species in California suggests an invasion into the high mountain ranges of the northeastern Mojave during the last interglacial via the Colorado River corridor. We explored the hypothesis that climate patterns shaped the current distribution of the Gila monster in California. Precipitation is decidedly biphasic east of 116° longitude, with over 24 percent falling in the warm season. Warm season precipitation data from recording stations closest to Gila monster localities are almost identical for those in western Arizona where the species is more common. Summer precipitation may be important in the foraging ecology of the species. Gila monsters were probably already rare in California long before the arrival of Europeans due to changes in climate and landform that delimited the marginal location of California in the range of this species. Fortunately, most of the habitat for this species in California is protected or relatively free from human disturbance.

Conservation genetics, evolution and distinct population segments of the Mojave fringe-toed lizard, Uma scoparia

Mojave fringe-toed lizards, Uma scoparia, occur only on desert sand dunes and associated mosaics of small sand ramps. Consequently, the distribution is naturally discontinuous and geographically complex. Many populations are vulnerable to local extirpation, and some are now gone. We synthesized genealogical and paleoecological data to explain the geographic relationships among populations of U. scoparia based on mitochondrial DNA sequence data. The species appears to have arisen from a common ancestor with U. notata by a vicariance event, associated with the formation and development of the Lower Colorado River. Within U. scoparia, two major maternal lineages are associated with the development of river drainage systems. The northern lineage is from the Amargosa River. The southerly lineage includes the Mojave River basin, Bristol Trough, Clark's Pass, and Colorado River sand transport systems. The northernmost lineage of U. scoparia, isolated in the vicinity of Plio-Pleistocene Lake Tecopa, has the greatest amount of DNA sequence divergence. The southern populations exhibit little local genetic differentiation. A small population near Red Pass Lake contains haplotypes from both the northern and southern lineages. We define two distinct population segments (DPSs), one associated with the Amargosa River lineage, and the other with Red Pass Dune.

Continental Physiography, Climate, and the Global Distribution of Human Population

Continental Physiography, Climate, and the Global Distribution of Human Population

Comparison of ground-water flow model particle-tracking results and isotopic data in the Mojave River ground-water basin, southern California, USA

Flow-path and time-of-travel results for the Mojave River ground-water basin, southern California, calculated using the ground-water flow model modflow and particle-tracking model modpath were similar to flow path and time-of-travel interpretations derived from delta-deuterium and carbon-14 data. Model and isotopic data both show short flow paths and young ground-water ages throughout the floodplain aquifer along most the Mojave River. Longer flow paths and older ground-water ages as great as 10,000 years before present were measured and simulated in the floodplain aquifer near the Mojave Valley. Model and isotopic data also show movement of water between the floodplain and regional aquifer and subsequent discharge of water from the river to dry lakes in some areas. It was not possible to simulate the isotopic composition of ground-water in the regional aquifer away from the front of the San Gabriel and San Bernardino Mountains—because recharge in these areas does not occur under the present-day climatic conditions used for calibration of the model.

A methodology to asess relations between climatic variability and variations in hydrologic time series in the southwestern United States

A new method for frequency analysis of hydrologic time series was developed to facilitate the estimation and reconstruction of individual or groups of frequencies from hydrologic time-series and facilitate the comparison of these isolated time-series components across data types, between different hydrologic settings within a watershed, between watersheds, and across frequencies. While climate-related variations in inflow to and outflow from aquifers have often been neglected, the development and management of ground-water and surface-water resources has required the inclusion of the assessment of the effects of climatic variability on the supply and demand and sustainability of use. The regional assessment of climatic variability of surface-water and ground-water flow throughout the southwestern United States required this new systematic method of hydrologic time-series analysis. To demonstrate the application of this new method, six hydrologic time-series from the Mojave River Basin, California were analyzed. The results indicate that climatic variability exists in all the data types and are partially coincident with known climate cycles such as the Pacific Decadal Oscillation and the El Nino–Southern Oscillation. The time-series also indicate lagged correlations between tree-ring indices, streamflow, stream base flow, and ground-water levels. These correlations and reconstructed time-series can be used to better understand the relation of hydrologic response to climatic forcings and to facilitate the simulation of streamflow and ground-water recharge for a more realistic approach to water-resource management.

Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity

We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW–SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located ∼32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system.A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including 14C activity) have measured helium concentrations that increase along flowpaths from 9.9×10−8 to 1.0×10−4 cm3 STP g−1 H2O. Concomitantly, 3He/4He ratios decrease from 0.84RA to 0.11RA (RA equals the 3He/4He ratio in air=1.4×10−6). We did not record 3He/4He ratios equivalent to crustal-production values (∼0.02RA) in any sample.Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The radiogenic He component has chronological significance, and good concordance between 4He and 14C ages for younger groundwaters (<25,000 year) demonstrates the integrity of the 4He-chronometer in this setting. Helium-rich waters could also be dated with the 4He technique, but only by first isolating the whole crustal flux (3–10×10−6 cm3 STP cm−2 year−1). Mantle-derived 3He (3Hem) is present in all MRB samples irrespective of distance from the SAF. However, regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived 3He in comparison with more modern samples. We propose that faults in the basin other than the SAF may be an additional source of mantle-derived helium. The large range in 3Hem concentrations may be related to fault activity; however, groundwaters with lower and more constant 3Hem contents may indicate that seismic activity along the SAF has been relatively constant for the past 30,000 years, demonstrating that ancient groundwaters may serve as an archive for paleo-seismic events.

Ground water flow in a desert basin: challenges of simulating transport of dissolved chromium.

A large chromium plume that evolved from chromium releases in a valley near the Mojave River was studied to understand the processes controlling fate and migration of chromium in ground water and used as a tracer to study the dynamics of a basin and range ground water system. The valley that was studied is naturally arid with high evapotranspiration such that essentially no precipitation infiltrates to the water table. The dominant natural hydrogeologic processes are recharge to the ground water system from the Mojave River during the infrequent episodes when there is flow in the river, and ground water flow toward a playa lake where the ground water evaporates. Agricultural pumping in the valley from the mid-1930s to the 1970s significantly altered ground water flow conditions by decreasing water levels in the valley by more than 20 m. This pumping declined significantly as a result of dewatering of the aquifer, and water levels have since recovered modestly. The ground water system was modeled using MODFLOW, and chromium transport was simulated using MT3D. Several innovative modifications were made to these modeling programs to simulate important processes in this ground water system. Modifications to MODFLOW include developing a new well package that estimates pumping rates from irrigation wells at each time step based on available drawdown. MT3D was modified to account for mass trapped above the water table when the water table declines beneath nonirrigated areas and to redistribute mass to the system when water levels rise.

Aeolian Destabilization Along the Mojave River, Mojave Desert, California: Linkages Among Fluvial, Groundwater, and Aeolian Systems

A notable aspect of ecosystem deterioration in arid regions is sand movement subsequent to the loss of plant cover. The Lower Mojave Valley in the Mojave Desert of southern California is undergoing rapid environmental change owing to groundwater drawdown, causing the death of phreatophytic vegetation and the reactivation of aeolian sand. Historical documentation shows that in the early twentieth century, sand derived from the open river channel accumulated to form large stabilized nebkhas, anchored by mesquite (Prosopis glandulosa), a plant that requires the water table to lie 9 m or less below the surface. The natural balance between winds, sand supply, and vegetation changed in the later twentieth century, as increasing demand for groundwater caused the water table to drop from a near-surface elevation to as much as 30 m below the surface. The mesquite that anchored the dunes declined in vigor or died. Riparian vegetation, including cottonwood and willow, was scoured from the channel during flood events, and did not recolonize as the low water table prevented the establishment of seedlings. Extensive new areas of the channel were laid open to wind erosion. Sand derived from the channel and cannibalized from the degrading nebkhas began to migrate downwind. Long sand stringers extended from west to east across the dunefield. In the 1990s, barchans developed atop the sand stringers, growing rapidly from incipient low-amplitude forms to well-developed barchans 60 m in width with sharply defined slipfaces 6 m in height. The dunefield has changed from a predominant mode of deposition to one of erosion and transportation, in which the water table and vegetation no long exert an influence. Dune encroachment from migrating sand streaks and dunes buried equipment, rendered buildings and homes unusable, blocked the entrances to property and destroyed pasture areas. Changes to the aeolian environment of the Lower Mojave Valley reflect complex interactions between the wind regime, river channel morphology and sediment, surface and subsurface hydrology, geology, vegetation, and human influence. This study demonstrates the sensitivity of aeolian systems to land-use changes, particularly in environments where wind is not a limiting factor, and illustrates the rapidity with which such changes can occur.

Movement of water through the thick unsaturated zone underlying Oro Grande and Sheep Creek Washes in the western Mojave Desert, USA

Previous studies indicate that a small quantity of recharge occurs from infiltration of streamflow in intermittent streams in the upper Mojave River basin, in the western Mojave Desert, near Victorville, California. Chloride, tritium, and stable isotope data collected in the unsaturated zone between 1994 and 1998 from boreholes drilled in Oro Grande and Sheep Creek Washes indicate that infiltration of streamflow occurs to depths below the root zone, and presumably to the water table, along much of Oro Grande Wash and near the mountain front along Sheep Creek Wash. Differences in infiltration at sites along each wash are the result of hydrologic variables such as proximity to the mountain front, quantity of streamflow, and texture of the subsurface deposits. Differences in infiltration between the washes are the result of large-scale geomorphic processes. For example, Oro Grande wash is incised into the Victorville fan and infiltration has occurred at approximately the same location over recent geologic time. In contrast, Sheep Creek Wash overlies an active alluvial fan and the stream channel can move across the fan surface through time. Infiltration does not occur to depths below the root zone at control sites outside of the washes.

The western pond turtle (Clemmys marmorata) in the Mojave River, California, USA: highly adapted survivor or tenuous relict?

AbstractAspects of the ecology of populations of the western pond turtle Clemmys marmorata were investigated in the Mojave River of the central Mojave Desert, California, U.S.A. One population occupied man‐made ponds and the other occurred in natural ponds in the flood plain of the Mojave River. Both habitats are severely degraded as a result of ground water depletion from human activities along the river and one is infested with the exotic shrub saltcedar Tamarix ramosissima. Mean female carapace length (CL) was significantly greater (14.4 cm) than that of males (13.7 cm). Live juveniles were not detected during the period of study. Shelled eggs were visible in X‐radiographs from 26 May to 14 July. Mean clutch size was 4.46 and ranged from 3 to 6 eggs. Clutch size did not vary between 1998 and 1999 but was significantly correlated with CL for both years combined, increasing at the rate of 0.548 eggs/cm CL. Gravid female CL ranged from 13.3–16.0 cm. Some females nested in both years. Mean X‐ray egg width (21.8 mm) was not significantly correlated with CL or clutch size. X‐ray egg width differed more among clutches than within, whether including CL as a co‐variate or not. Nesting migrations occurred from 6 June to 8 July with minimum round trip distances ranging from 17.5–585 m with a mean of 195 m. Mean estimated time of departure as measured at the drift fence was 18:13. Most females returned to the ponds in the early morning. Nesting migrations required females to be out of the water for estimated periods of 0.83 to 86 h. The destination of nesting females was typically fluvial sand bars in the channel of the dry riverbed. Overall, the ecology of C. marmorata in the Mojave River is very similar to that reported for populations in less severe habitats along the west coast of the United States. Notable exceptions include long nesting migrations to sandbars in the dry river channel, a possible result of human modifications to the environment, and an apparent lack of terrestrial overwintering behaviour in Mojave River populations. The general similarity of desert and coastal populations is possibly a reflection of their recent geographic separation. Overall, populations in the Mojave River exhibit few obvious adaptations to living in the desert and are considered to be tenuous relicts of the Pleistocene. The small size and tenuous status of these populations suggests that immediate conservation action is needed, including establishment of satellite populations as a hedge against extirpation.

Water movement through a thick unsaturated zone underlying an intermittent stream in the western Mojave Desert, southern California, USA

Previous studies indicated that small amounts of recharge occur as infiltration of intermittent streamflow in washes in the upper Mojave River basin, in the western Mojave Desert, near Victorville, California. These washes flow only a few days each year after large storms. To reach the water table, water must pass through an unsaturated zone that is more than 130 m thick. Results of this study, done in 1994–1998, show that infiltration to depths below the root zone did not occur at control sites away from the wash. At these sites, volumetric water contents were as low as 0.01 and water potentials (measured as the combination of solute and matric potentials using a water activity meter) were as negative as −14,000 kPa. Water-vapor movement was controlled by highly negative solute potentials associated with the accumulation of soluble salts in the unsaturated zone. Highly negative matric potentials above and below the zone of maximum solute accumulation result from movement of water vapor toward the highly negative solute potentials at that depth. The δ 18O and δD (delta oxygen-18 and delta deuterium) isotopic composition of water in coarse-grained deposits plots along a Rayleigh distillation line consistent with removal of water in coarse-grained layers by vapor transport. Beneath Oro Grande Wash, water moved to depths below the root zone and, presumably, to the water table about 130 m below land surface. Underneath Oro Grande Wash, volumetric water contents were as high as 0.27 and water potentials (measured as matric potential using tensiometers) were between −1.8 and −50 kPa. On the basis of tritium data, water requires at least 180–260 years to infiltrate to the water table. Clay layers impede the downward movement of water. Seasonal changes in water vapor composition underneath the wash are consistent with the rapid infiltration of a small quantity of water to great depths and subsequent equilibration of vapor with water in the surrounding material. It may be possible to supplement natural recharge from the wash with imported water. Recharge to the wash may be advantageous because the unsaturated zone is not as dry as most areas in the desert and concentrations of soluble salts are generally lower underneath the wash.

Channel incision and patterns of cottonwood stress and mortality along the Mojave River, California

In 1995, mapping and classification of riparian vegetation along the Mojave River in southern California revealed an 8-km reach in which riparian cottonwoods (Populus fremontii Wats.) were stressed or dying. We tested a set of predictions based on the inference that cottonwood decline was an indirect result of lowered water-table levels following flood-related channel incision. Comparisons of topographic cross-sections from 1963 and 1997, indicated a net change in channel elevation between −0·71 and −3·6 m within zones of cottonwood stress and mortality. Ages of young cottonwood and willow stems adjacent to the present channel and radial stem growth of surviving cottonwoods were consistent with the inference that channel incision, associated with sustained flooding in January and February of 1993, lowered channel elevations throughout the affected reach. Well records and soil redoximorphic features indicate that channel incision caused net water-table declines ⩾1·5 m on portions of the adjacent flood plain where cottonwood stand mortality ranged between 58 and 93%. In areas where water-table declines were estimated to be <1·0 m, stand mortality was 7–13%.

Aeolian system sediment state: theory and Mojave Desert Kelso dune field example

The sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply, sediment availability and the transport capacity of the wind. The sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system. Numerous factors impact the susceptibility of grains on a surface to transport, but these are cumulatively manifested by the actual transport rate, which serves as a proxy for sediment availability. Transport capacity is the potential sediment transport rate of the wind. Because the three aspects of sediment state can be given as a volumetric rate, they are directly comparable. Plotted simultaneously against time, the generated curves define nine possible classes of sediment state. Sediment supply that is stored occurs because it is transport or availability limited, or generated at a rate greater than the potential or actual transport rates respectively. Contemporaneous or lagged influx to an aeolian system may be limited by sediment availability, but cannot exceed the transport capacity of the wind. For the Kelso dune field in the Mojave Desert of California, a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system. The sediment supply was generated during the latest Pleistocene and earliest Holocene during humid periods of enhanced discharge by the Mojave River to form the Lake Mojave fan delta or terminal fan, and has been calculated over time from the sedimentation rate and the frequency of floods. Estimation of transport capacity over time was based upon modern wind data, with an allowance for greater winds during the Pleistocene based upon climatic models. Sediment availability was approximated by calculation of a modern dune mobility index, with variation over time based upon climatic inferences. While quantifying the Kelso or any natural system is subject to numerous uncertainties, the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at Kelso, as well as illuminating questions for future research.

Identification of sand sources and transport pathways at the Kelso Dunes, California, using thermal infrared remote sensing

The Kelso dune field is located in the eastern Mojave Desert, California, at the terminus of a sand-transport pathway, which has its primary source at the Mojave River Wash 50 km to the west. Initial examination of 1984 airborne thermal infrared multispectral scanner (TIMS) data showed significant spectral variations that indicate potential mineralogic heterogeneities within the active dunes. This result prompted the collection of a suite of 48 sand samples in 1990, and the acquiring of new TIMS data in 1995. This new data set was used to test a newly developed linear spectral retrieval algorithm in conjunction with a spectral library of end-member minerals. Results of this analysis produced images of end-member minerals that showed marked variations within the dunes. In addition, standard petrographic techniques revealed that the dunes contain mineralogic variations and were much less quartz rich (∼42%) than previously reported (∼70%–90%). Point-count results agreed with the spectral data to within an average of 5.3% for TIMS-derived and 3.1% for laboratory-derived mineralogic abundances. High concentrations of several of the end-member minerals on the surrounding alluvial fans indicate a potential nearby source for these minerals. Most evident is the presence of potassium feldspar from the fan that emanates from the Providence Mountains east of the dunes. This previously unidentified potential sand input is not visible with other remote sensing techniques and was confirmed with additional field sampling. Much of the quartz and some plagioclase feldspar, however, appear to have been transported from the Mojave River Wash source as previously reported. This study also validates the potential of using thermal remote sensing from future satellite-based instruments to globally monitor desert fringe areas susceptible to the changing conditions of sand encroachment.

Extracting Holocene paleohydrology and paleoclimatology information from modern extreme flood events: An example from southern California

The extraction of paleohydrological and paleoclimatological information from a modern hydrological system, shown to represent unique and extreme hydroclimatological conditions, is illustrated by an example from the Mojave River drainage basin in southern California. The Mojave River allows only the most extreme floods to reach its terminal basin in the Silver Lake playa and to form ephemeral lakes. All the other floods are lost by transmission into the alluvial aquifer along its 200 km channel. This filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions. We demonstrate such a link between anomalous, present-day atmospheric circulation patterns over the North Pacific Ocean, extreme storms in southern California that produced the heaviest precipitation on record, the largest floods of record in the Mojave River watershed, and ephemeral lakes in its terminal playa. This physical link determines the possible cause of the formations of perennial, short-duration, shallow lakes in Silver Lake playa during the late Holocene and characterizes the hydroclimatic conditions that prevailed during these lacustrine episodes. Hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modern storms and floods were more frequent in at least an order of magnitude during specific time episodes. We conclude that such extreme hydroclimatic conditions occurred more frequent in past episodes during which the Holocene lakes formed. In turn, this conclusion indicates that the cause of these storms and floods, i.e. the anomalous atmospheric circulation pattern, must have been more frequent. This research outlines a way to extract information on Holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship.