Topographic Regimes Research Articles

Experiences with the RTM Method in Local Quasi-Geoid Modeling

In local quasi-geoid modeling, the residual terrain modeling (RTM) method is often used to remove short-wavelength gravity field signals from the measured gravity on the ground in order to obtain a regularized and smooth gravity field that is suited for field interpolation and modeling. Accurate computation of RTM corrections plays a crucial role in computing an accurate local quasi-geoid, and it requires a set of fine-tuned parameters, including the combination of DEMs with different resolutions for suitably representing the real topography, the choice of integration radius for properly defining the extent of the computation zone, and the determination of reference topography to properly describe the RTM-reduced Earth’s surface. To our knowledge, this has not been systematically documented, despite its obvious importance. This study aims to systematically investigate the impact of these factors on RTM correction computation and, consequently, on local quasi-geoid modeling to provide practical guidelines for real-world applications. The tesseroid-based gravity forward modeling technique is employed to investigate the following issues existing in the practical use of the RTM method: ① Can the combination of a high-resolution DEM and a DEM with a lower resolution replace the single use of the high-resolution DEM for RTM correction computation while maintaining accuracy and improving efficiency? If it does, how do I properly choose the resolution of this coarse DEM as well as the integration radius r1 for the inner zone and r2 for the outer zone? ② How large would the differences between the RTM corrections computed by three types of reference topographies, which are obtained from the direct averaging (DA) approach, the moving averaging (MA) approach, and the spherical harmonic (SH) approach, be, and how large would their impact on quasi-geoid modeling be? To obtain objective findings, two research regions were selected for this investigation. One is the Colorado test area (USA) with rugged terrain, and the other is the Auvergne test area (France) with moderate terrain. The main numerical findings are: (1) the combination of the 3” resolution DEM (inner zone) and the 30″ resolution DEM (outer zone) is sufficient for accurate and efficient RTM correction computation; (2) if the resolution of the reference topography is 5′ or slightly lower, all three types of reference topographies are able to obtain local quasi-geoid models at a similar accuracy level, while the values of r1 and r2 are preferred to be at least 20 km and 111 km, respectively; (3) if the reference topography has a resolution of 30′ or lower, the MA or SH reference topography is recommended, especially for the latter one, and the values of r1 and r2 are suggested to be at least 20 km and 222 km, respectively. The above numerical findings can be taken as a reference for local quasi-geoid determination in areas with different topographic regimes than the two selected test areas.

Energetics and Mixing of Stratified, Rotating Flow over Abyssal Hills

Abstract One of the proposed mechanisms for energy loss in the ocean is through dissipation of internal waves, in particular above rough topography where internal lee waves are generated. Rates of dissipation and diapycnal mixing are often estimated using linear internal wave generation theory and a constant value for mixing efficiency. However, previous oceanographic measurements found that nonlinear dynamics may be important close to topography. To investigate the role of nonlinear interactions, we conduct idealized 3D direct numerical simulations (DNS) of steady flow over 1D topography and vary the topographic height, which correlates to the degree of flow nonlinearity. We analyze the spatial distribution of energy transfer rates between internal waves and the nongeostrophic portion of the time-mean flow, and of dissipation and diapycnal mixing rates. In our simulations with taller, more nonlinear topographies, energy transfer rates are similar to previously unexplained oceanographic observations near topography: internal waves gain energy from time-mean flow through horizontal straining and lose energy through vertical shearing. In the tall topography simulations, buoyancy fluxes also play a significant role, consistent with observations but contrary to linear wave theory, suggesting that quasigeostrophy-based approximations and linear theory may not hold in some regions above rough topography. Both dissipation and mixing rates increase with topographic height, but their vertical distributions differ between topographic regimes. As such, the vertical profile of mixing efficiency is different for linear and nonlinear topographic regimes, which may need to be incorporated into parameterizations of small-scale processes in models and estimates of ocean energy loss.

The RTM harmonic correction revisited

In this paper, we derive improved expressions for the harmonic correction to gravity and, for the first time, expressions for the harmonic correction to potential and height anomaly. They need to be applied at stations buried inside the masses to transform internal values into harmonically downward continued values, which are then input to local quasi-geoid modelling using least-squares collocation or least-squares techniques in combination with the remove-compute-restore approach. Harmonic corrections to potential and height anomaly were assumed to be negligible so far resulting in yet unknown quasi-geoid model errors. The improved expressions for the harmonic correction to gravity, and the new expressions for the harmonic correction to potential and height anomaly are used to quantify the approximation errors of the commonly used harmonic correction to gravity and to quantify the magnitude of the harmonic correction to potential and height anomaly. This is done for two test areas with different topographic regimes. One comprises parts of Norway and the North Atlantic where the presence of deep, long, and narrow fjords suggest extreme values for the harmonic correction to potential and height anomaly and corresponding large errors of the commonly used approximation of the harmonic correction to gravity. The other one is located in the Auvergne test area with a moderate topography comprising both flat and hilly areas and therefore may be representative for many areas around the world. For both test areas, two RTM surfaces with different smoothness are computed simulating the use of a medium-resolution and an ultra-high-resolution reference gravity field, respectively. We show that the errors of the commonly used harmonic correction to gravity may be as large as the harmonic correction itself and attain peak values in areas of strong topographic variations of about 100 mGal. Moreover, we show that this correction may introduce long-wavelength biases in the computed quasi-geoid model. Furthermore, we show that the harmonic correction to height anomaly can attain values on the order of a decimetre at some points. Overall, however, the harmonic correction to height anomaly needs to be applied only in areas of strong topographic variations. In flat or hilly areas, it is mostly smaller than one centimetre. Finally, we show that the harmonic corrections increase with increasing smoothness of the RTM surface, which suggests to use a RTM surface with a spatial resolution comparable to the finest scales which can be resolved by the data rather than depending on the resolution of the global geopotential model used to reduce the data.

Thar Desert Bioresources: Significance, Conservation and Sustainable Management in Anthropocene

‘Anthropocene’, the era of earth’s history during which human ( Homo sapiens ) have become a dominant geological force. The anthropogenic activities have irreversibly impacted the globe, the atmosphere, and therefore the climate system as a whole. The global warmings associated with climate changes and other constraints such as small landholdings, lower land productivity, water stress, especially in drylands increased vulnerabilities to the local inhabitants. Indian Thar Desert, a unique stressed ecosystem, harbours diverse life-forms (plants, animals and microbes) having acquired mechanisms of survival under fragile conditions (biotic and abiotic stress, especially recurring droughts). These extremes in the climatic, edaphic and topographic regimes provided the exceptional natural environmental conditions for origin, growth, adaptation and evolution of variety of life-forms. Desert specific species harbours traits/genes such as heat, drought, salt tolerance, etc. that may be important to answer the challenges being caused by anthropogenic disturbances. The natural resource management through agroecology and agroforestry systems practiced by the native people is the best option as an insurance in case of crop failure due to environmental circumstances. In this paper, we review the present scenario of biological diversity of desert, human induced/anthropogenic threats/constraints to arid/semi-arid ecosystem and geo-ecological parameters. The possible solutions to some of these problem through intervention of innovative technologies are also discussed.

Sentinel-3 Delay-Doppler altimetry over Antarctica

Abstract. The launch of Sentinel-3A in February 2016 represented the beginning of a new long-term series of operational satellite radar altimeters, which will provide Delay-Doppler altimetry measurements over ice sheets for decades to come. Given the potential benefits that these satellites can offer to a range of glaciological applications, it is important to establish their capacity to monitor ice sheet elevation and elevation change. Here, we present the first analysis of Sentinel-3 Delay-Doppler altimetry over the Antarctic ice sheet, and assess the accuracy and precision of retrievals of ice sheet elevation across a range of topographic regimes. Over the low-slope regions of the ice sheet interior, we find that the instrument achieves both an accuracy and a precision of the order of 10 cm, with ∼98 % of the data validated being within 50 cm of co-located airborne measurements. Across the steeper and more complex topography of the ice sheet margin, the accuracy decreases, although analysis at two coastal sites with densely surveyed airborne campaigns shows that ∼60 %–85 % of validated data are still within 1 m of co-located airborne elevation measurements. We then explore the utility of the Sentinel-3A Delay-Doppler altimeter for mapping ice sheet elevation change. We show that with only 2 years of available data, it is possible to resolve known signals of ice dynamic imbalance and to detect evidence of subglacial lake drainage activity. Our analysis demonstrates a new, long-term source of measurements of ice sheet elevation and elevation change, and the early potential of this operational system for monitoring ice sheet imbalance for decades to come.

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

AbstractTerrestrial gradients in the oxygen isotopic composition of meteoric water (δ18O), as reconstructed through proxies, reflect characteristics of ancient hydrologic conditions. These gradients are primarily influenced by the atmospheric transport of water vapor and the balance of precipitation and evapotranspiration, which are linked to climate and topography. We incorporate these effects into a one‐dimensional model that predicts the spatial evolution of δ18O based on local topography and the regional water‐energy budget. Specifically, we build on existing reactive transport models by incorporating parameterizations of orographic precipitation and energetic constraints on evapotranspiration following the Budyko water balance framework. We test our model on three modern transects that represent topographically distinct environments. These are the Amazon Basin (lowlands), the Cascade Range (mountains), and the eastern Himalayan Range (lowlands and mountains). Comparisons among these gradients demonstrate that the topographic regime determines how sensitive isotope records are to hydroclimate evolution. As a result, isotope records differentially express signatures of topography and the regional water balance, and we present a quantitative framework to predict this trade‐off. Finally, we link these effects to climate evolution and discuss how our model may help disentangle topographic and climatic signals through Earth history.

Assessment and Comparison of TMPA Satellite Precipitation Products in Varying Climatic and Topographic Regimes in Morocco

TRMM Multi-satellite Precipitation Analysis (TMPA) satellite precipitation products have been utilized to quantify, forecast, or understand precipitation patterns, climate change, hydrologic models, and drought in numerous scientific investigations. The TMPA products recently went through a series of algorithm developments to enhance the accuracy and reliability of high-quality precipitation measurements, particularly in low rainfall environments and complex terrain. In this study, we evaluated four TMPA products (3B42: V6, V7temp, V7, RTV7) against 125 rain gauges in Northern Morocco to assess the accuracy of TMPA products in various regimes, examine the performance metrics of new algorithm developments, and assess the impact of the processing error in 2012. Results show that the research products outperform the real-time products in all environments within Morocco, and the newest algorithm development (3B42 V7) outperforms the previous version (V6), particularly in low rainfall and high-elevation environments. TMPA products continue to overestimate precipitation in arid environments and underestimate it in high-elevation areas. Lastly, the temporary processing error resulted in little bias except in arid environments. These results corroborate findings from previous studies, provide scientific data for the Middle East, highlight the difficulty of using TMPA products in varying conditions, and present preliminary research for future algorithm development for the GPM mission.

Comparison of NCEP Multisensor Precipitation Estimates with Independent Gauge Data over the Eastern United States

AbstractGauge-calibrated radar estimates of daily precipitation are compared with daily observed values of precipitation from National Weather Service (NWS) Cooperative Observer Network (COOP) stations to evaluate the multisensor precipitation estimate (MPE) product that is gridded by the National Centers for Environmental Prediction (NCEP) for the eastern United States (defined as locations east of the Mississippi River). This study focuses on a broad evaluation of MPE across the study domain by season and intensity. In addition, the aspect of precipitation type is considered through case studies of winter and summer precipitation events across the domain. Results of this study indicate a north–south gradient in the error of MPE and a seasonal pattern with the highest error in summer and autumn and the lowest error in winter. Two case studies of precipitation are also considered in this study. These case studies include instances of intense precipitation and frozen precipitation. These results suggest that MPE is less able to estimate convective-scale precipitation as compared with precipitation variations at larger spatial scales. In addition, the results suggest that MPE is subject to errors related both to the measurement gauges and to the radar estimates used. Two case studies are also included to discuss the differences with regard to precipitation type. The results from these case studies suggest that MPE may have higher error associated with estimating the liquid equivalent of frozen precipitation when compared with NWS COOP network data. The results also suggest the need for more analysis of MPE error for frozen precipitation in diverse topographic regimes.

Seasonal dynamics, environmental preferences and habitat selection of John Dory ( Zeus faber)

Spatial distribution patterns of John Dory ( Zeus faber, L.) were examined in relation to sea bottom temperature, bathymetry, locational covariates and season. Data were collected during a 2-year period (1996–1997) of seasonal sampling using demersal trawl surveys in the Aegean Sea (eastern Mediterranean). The ecological preferences of the species throughout four seasons were elucidated and the degree in which these environmental relationships might be modulated by the different hydrographic and topographic regime was also considered. Key determinants of the species' spatial aggregation in all four seasons were the water depth, the sea bottom temperature and the latitude. The sea bottom temperature had a significant effect on seasonal John Dory abundance both directly, as a main effect, and indirectly, through its interactive effect with the water depth. John Dory seasonal abundance was consistently greater in the shallower regions of the area having warmer bottom waters. Results indicated a distinct southward shift as progressing through the year, with peak abundances being observed in gradually lower latitudes of shallow areas as seasons evolved. John Dory appeared to avoid the deeper waters regardless of their bottom temperature and geographic position. The present results also suggested a seasonal pattern of habitat associations for specific locations characterised by weak hydrographic activity. John Dory preferences for certain water depth zones, sea bottom temperatures and substrate types are hypothesized to modulate the seasonal spatial aggregation of the species to preferred grounds.

Synoptic and Topographic Variability of Northern California Precipitation Characteristics in Landfalling Winter Storms Observed during CALJET

Abstract Observations from northern California during the California Landfalling Jets (CALJET) experiment are used to examine the mean characteristics of precipitation and their variances as functions of synoptic and topographic regimes. Ten cases involving the landfall of extratropical cyclones are analyzed with radar and rain gauge data collected at two sites: one in the coastal mountains north of San Francisco (CZD) and the other in the Central Valley just west of Sacramento (KDAX). Aside from the melting-layer bright band, the most striking feature in the 10-case composite vertical profile of radar reflectivity at CZD was a distinct change in slope about 2.5 km above the bright band. This “shoulder” is thought to represent a change in the growth rate of hydrometeors. Although the bright band was quite distinct, about one-third of the profiles in the composite did not exhibit this feature. These nonbrightband (NBB) profiles had a low-level slope where reflectivity increased with decreasing altitude, a structure suggesting that collision–coalescence was the primary growth process. The relationship between surface rainfall rate and low-level radar reflectivity implies that all profiles were composed of larger numbers of small drops than expected from a Marshall–Palmer drop size distribution, a trend that was especially apparent for NBB profiles. Synoptic variability of precipitation characteristics at CZD were examined by identifying five distinct regimes (cold sector, warm front, warm sector, cold front, and cool sector) based on a simplified conceptual model. The shoulder remained approximately 2.5 km above the bright band in each regime. Rainfall intensity was highest during the cold-frontal regime and NBB rainfall was most common during the warm-frontal, warm-sector, and cool-sector regimes. Topographic variability of precipitation characteristics was investigated by comparing results at CZD and KDAX. A shoulder structure located about 2.5 km above the bright band was also evident in the KDAX profiles, suggesting that this feature is related to large-scale dynamic, thermodynamic, and microphysical processes rather than orographic effects. The relationship between surface rainfall rate and low-level radar reflectivity near KDAX closely followed a trend expected for a Marshall–Palmer drop size distribution, implying the presence of relatively larger raindrops than observed at CZD and indicating that NBB rainfall occurs less frequently near KDAX.

Comparison of methods to model the gravitational gradients from topographic data bases

A number of methods have been developed over the last few decades to model the gravitational gradients using digital elevation data. All methods are based on second-order derivatives of the Newtonian mass integral for the gravitational potential. Foremost are algorithms that divide the topographic masses into prisms or more general polyhedra and sum the corresponding gradient contributions. Other methods are designed for computational speed and make use of the fast Fourier transform (FFT), require a regular rectangular grid of data, and yield gradients on the entire grid, but only at constant altitude. We add to these the ordinary numerical integration (in horizontal coordinates) of the gradient integrals. In total we compare two prism, two FFT and two ordinary numerical integration methods using 1″ elevation data in two topographic regimes (rough and moderate terrain). Prism methods depend on the type of finite elements that are generated with the elevation data; in particular, alternative triangulations can yield significant differences in the gradients (up to tens of Eötvös). The FFT methods depend on a series development of the topographic heights, requiring terms up to 14th order in rough terrain; and, one popular method has significant bias errors (e.g. 13 Eötvös in the vertical–vertical gradient) embedded in its practical realization. The straightforward numerical integrations, whether on a rectangular or triangulated grid, yield sub-Eötvös differences in the gradients when compared to the other methods (except near the edges of the integration area) and they are as efficient computationally as the finite element methods.

Morphological regions and oblique-incidence dot formation in a model of surface sputtering

We study solid surface morphology created by off-normal ion-beam sputtering with an atomistic, solid-on-solid model of sputter erosion. With respect to an earlier version of the model, we extend this model with the inclusion of lateral erosion. Using the 2-dimensional structure factor, we found an upper bound $\mu\simeq 2$, in the lateral straggle $\mu$, for clear ripple formation. Above this upper bound, for longitudinal straggle $\sigma\gtrsim 1.7$, we found the possibility of dot formation (without sample rotation). Moreover, a temporal crossover from a hole topography to ripple topography with the same value of collision cascade parameters was found. Finally, a scaling analysis of the roughness, using the consecutive gradient approach, yields the growth exponents $\beta=0.33$ and 0.67 for two different topographic regimes.

The Quasi-Nondispersive Regimes of Long Extratropical Baroclinic Rossby Waves over (Slowly Varying) Topography

Abstract Actual energy paths of long, extratropical baroclinic Rossby waves in the ocean are difficult to describe simply because they depend on the meridional-wavenumber-to-zonal-wavenumber ratio τ, a quantity that is difficult to estimate both observationally and theoretically. This paper shows, however, that this dependence is actually weak over any interval in which the zonal phase speed varies approximately linearly with τ, in which case the propagation becomes quasi-nondispersive (QND) and describable at leading order in terms of environmental conditions (i.e., topography and stratification) alone. As an example, the purely topographic case is shown to possess three main kinds of QND ray paths. The first is a topographic regime in which the rays follow approximately the contours f /hαc = a constant (αc is a near constant fixed by the strength of the stratification, f is the Coriolis parameter, and h is the ocean depth). The second and third are, respectively, “fast” and “slow” westward regimes little affected by topography and associated with the first and second bottom-pressure-compensated normal modes studied in previous work by Tailleux and McWilliams. Idealized examples show that actual rays can often be reproduced with reasonable accuracy by replacing the actual dispersion relation by its QND approximation. The topographic regime provides an upper bound (in general a large overestimate) of the maximum latitudinal excursions of actual rays. The method presented in this paper is interesting for enabling an optimal classification of purely azimuthally dispersive wave systems into simpler idealized QND wave regimes, which helps to rationalize previous empirical findings that the ray paths of long Rossby waves in the presence of mean flow and topography often seem to be independent of the wavenumber orientation. Two important side results are to establish that the baroclinic string function regime of Tyler and Käse is only valid over a tiny range of the topographic parameter and that long baroclinic Rossby waves propagating over topography do not obey any two-dimensional potential vorticity conservation principle. Given the importance of the latter principle in geophysical fluid dynamics, the lack of it in this case makes the concept of the QND regimes all the more important, for they are probably the only alternative to provide a simple and economical description of general purely azimuthally dispersive wave systems.

Low-frequency variability of sea level and currents off Newfoundland

TOPEX/Poseidon (T/P) sea level data for the period from 1992 to 2002 are analysed to investigate sea level and current variability over the Newfoundland Slope. Sea level anomalies relative to local means for this period are derived and surface current anomalies normal to ground tracks are then calculated under the geostrophic approximation. Climatological mean surface currents normal to the ground tracks are also obtained from the solution of a regional circulation model. The sum of the model means and altimetric current anomalies is used to nominally represent absolute currents. Statistical analyses are carried out based on topographic regimes for along-slope and cross-slope current variability. The altimetric sea level and current variability increases offshore, toward the Gulf Stream or its extension the North Atlantic Current. Typical cross-track current variability is 10–20 cm/s over the southwestern (SW) and northeastern (NE) Slopes and 20–30 cm/s over the southeastern (SE) Slope. The total current variability may be estimated by a factor of 1.5 from the cross-track current variability of either the descending or ascending tracks. The sea level was higher in summer and lower in winter, with an increased range offshore. The shelf-edge current (water depth of 200–3000 m) was stronger in fall and weaker in spring. The seasonal range was up to 10–20 cm/s. Interannual sea level and current variations were also substantial. The sea level over the SW Newfoundland Slope had a fall from 1994 to 1996 and a rebound after 1997, more significant towards the deep sea, probably related to the north–south movement of the Gulf Stream. The NE Slope featured a sea level rise on the lower slope over the study period, associated with the freshening of the subpolar Northwest Atlantic. Patterns over the SE Slope appear to be in between those of the SW and NE Slopes. The Labrador Current over the slope (water depth of 200–3000 m) seemed to be strongest in 1996–1997, as a result of the equatorward traveling Labrador Current pulse.

Characteristics of rapid giant landslides in China

Factual data for 70 rapid, giant landslides since 1900 show that the occurrence of these landslides was largely predisposed by tectonics, geological structures, lithology and topography, and often triggered by rainfall and earthquakes. In terms of mobile behavior, the giant landslides can be classified into three types: slides, slide-flows and flows. It is found that each type of landslide was constrained to certain geologic and topographic regimes. There are good correlations between kinematic parameters of landslides and slope geometries, which confirm the important role played by topographical condition in the mobile behavior of landslides. Moreover, it is also found that each type of landslide presents distinct geotechnical characteristics in terms of nature of the slip zone and properties of sliding mass. Brief analyses of five typical cases illustrate that landslide mechanisms can be conceptually depicted by failure mechanisms of their slip zones prior to onset of movement and following energy conversion during movement. Problems and questions related to experience in China suggest that comprehensive and systematic investigation and study on rapid giant landslides are urgently needed.

Geology and stratigraphy of King crater, lunar farside

Clementine and photographic data sets have been used to investigate the crustal stratigraphy and geology of King crater on the lunar farside (120°E, 5.5°N). Pre-existing topographic regimes or stress fields dominate many structures in the crater, which has excavated materials from depths of up to 14 km. The upper crust in the area is noritic anorthosite, grading to a more anorthositic signature with depth. A possible batholithic intrusion is also present in a 15-km-wide band, extending from the southern crater floor to at least 50 km north of King, and from near-surface levels down to at least the excavation depth of the crater. It is generally feldspathic, but is cut by mafic dykes now visible in the north wall. King also shows evidence for the presence of a cryptomare, exposed in regions of the peaks and in dark halo craters within the ejecta blanket. Localized olivine-bearing mineralogies are observed on the central peaks, suggesting isolated pockets of troctolitic mineralogies to have been present at 8- to 14-km depths. Copious volumes of crystalline melt produced from the impact event cover King’s floor to a maximum thickness of 30–60 m, and have pooled in a number of natural depressions outside of the main crater. The main pool in the pre-existing A1-Tusi crater has a minimum depth of 150 m. Domes on the crater floor are verified as nonvolcanic in origin, and did not act as a source for any of the lava-like materials in King.

Use of airborne lidar to determine aerosol sources and movement in the Lower Fraser Valley (LFV), BC

A major oxidants field experiment (Pacific '93) was carried out in Vancouver, British Columbia, in July/August 1993. The purpose of the experiment was to provide data on the three-dimensional extent and movement of pollutants in a complex topographic regime so that predictive pollutant models could be assessed. As part of the study, 1.064 μm downward pointing lidar was flown on the National Research Council of Canada (NRCC) Convair 580 aircraft. These data have been used to visualize the aerosol pollutant structure throughout the lower Fraser Valley. While the majority of the pollution in the valley is from the urbanised sector around Vancouver, there are at least seven additional point source emitters which impact the valley in a significant way. The data are compared with in situ particle probe data from the aircraft when flying at low level and with surface sensors. Aerosol backscatter ratio at 500 m altitude is about 25–60% larger than predictions from a Mie scattering model using the in situ particle probe size distributions.

Was There a Pliocene-Pleistocene Fluvial-Lacustrine Connection between Death Valley and the Colorado River?

Since the turn of the century, a Pliocene-Pleistocene connection between the Death Valley-Owens River pluvial system and the Colorado River drainage basin has been frequently postulated. The two most commonly proposed routes involve (1) a southward overflow from the Death Valley Lake system or (2) southward migration of the Mojave River between its present course and a more southerly route. Under the present topographic regime, a Death Valley Lake capable of overflowing the bedrock saddle at Ludlow, California and discharging southward into the Bristol, Cadiz, and Danby Lake basins (and eventually the Colorado River) would be over 12,000 km 2 in size. Few surface and subsurface indicators exist to support either a fluvial or lacustrine connection. Evidence from deep cores and boreholes drilled in Soda, Bristol, Cadiz, and Danby dry lake basins indicate that a hydrologic connection has not occurred during the past 4 myr. No well-documented paleoshoreline features have been located at elevations corresponding to the above hydrologic systems in Death Valley, Silver-Soda, Bristol, or Danby Lake basins. In the Cadiz, Silurian, and Broadwell basins these features have not been found at all. Therefore, we conclude that a hydrologic connection between the Death Valley-Owens River system and the Colorado River has not occurred along either of these routes since the middle Pliocene.