Allen H. Christensen

New field method to determine streamflow timing using electrical resistance sensors

can become buried or damaged by moving sediment or debris. Consequently, streamflow timing sensors deElectrical resistance (ER) sensors were constructed to monitor ployed within the vadose zone have been shown to be streambed saturation to infer ephemeral streamflow timing. The senadvantageous under these circumstances (Constantz et sors were evaluated in an ephemeral stream through comparison with temperature-based methods, a stream gauge, and soil water content al., 2001). sensors. The ER sensors were more accurate at estimating streamflow Soil water content methods detect infiltration and timing and the resultant data required less interpretation than data percolation of water through the sediments, which may from temperature-based methods. Accuracy was equivalent to timing be used to infer timing of streamflow (Blasch et al., unmethods using stream gauge and soil water content measurements. published data). Placing sensors in the subsurface reThe ER sensors are advantageous for use in ephemeral stream chanduces the possibility that they will be damaged or lost nels because they are inexpensive, deployable above or below the during flow. Logging instrumentation, however, must sediment surface, insensitive to depth, and do not require connecting be placed on or near the bank with cables extending to wires to an external datalogger or power source. On the basis of these the buried sensors. results, we conclude that ER sensors may be used to monitor changes Temperature methods enable inference of streamflow in soil water content within the vadose zone. Additionally, the sensors can be used to infer the presence of surface water in diversion canals, timing on the basis of the combined transport of heat and storm-water sewers, and in the form of overland runoff. fluid within the bed sediments (Constantz and Thomas, 1996, 1997; Ronan et al., 1998; Constantz et al., 2001). Recent development of small ( 10 cm3), inexpensive, waterproof temperature sensors with integrated data G the erratic and variable nature of ephemeral and intermittent streamflow in arid and semiarid storage enable measurement and storage of temperature values without the need for external connecting basins, long-term collection of streamflow timing is necessary for obtaining information on extreme flow events wires. This advantage enables in situ temperature monitoring in ephemeral channels with unstable beds over and seasons. Streamflow timing in channels and arroyos is used to accurately model fluid transport through the large areas with high spatial resolution. While temperature methods have been used successunsaturated zone beneath ephemeral streams and to constrain channel recharge estimation, a primary compofully to monitor the timing of streamflow in ephemeral channels, the methods have limitations. Specifically, cernent of aquifer replenishment. Additionally, streamflow timing is a necessary component for designing stormtain conditions are required for streamflow to produce a readily identifiable thermal signal. For example, water water and flood-control networks in flood-prone environments. with the same temperature as the channel will not produce an identifiable signal. In ephemeral stream chanCurrent methods used to estimate streamflow timing include flow-rated stream gauges, velocity meters, soil nels subject to repeated scour and deposition, changes in sediment surface elevation complicate the application water content sensors, and temperature sensors (Latkovich and Leavesly, 1993; Constantz et al., 2001; Blasch of numerical methods used for interpretation of the temperature data. et al., unpublished data). These methods have met with varying success depending upon channel morphology, In this investigation, we converted commercially available temperature sensors into ER sensors to monitor bed sediment characteristics, frequency and duration of streamflow, and other requirements (e.g., magnitude of water content and tested their utility for streamflow detection. Advantages of the ER approach include functemperature signal). Stream gauges and velocity meters accurately detertionality above or below the channel surface, functionality in all streamflow temperatures, lack of connecting mine streamflow timing, but generally are not suitable for ephemeral channels that experience changes in chanwires, and minimal interpretation of data. These same attributes necessary for streamflow timing are also adnel morphology (Tadayon et al., 2000). Stream gauges and velocity meters installed at the bed sediment surface vantageous for monitoring sediment saturation in other similar vadose zone applications such as irrigated fields, fluctuating water tables, and postburn environments. K. Blasch, U.S. Geological Survey, 520 North Park Ave., Suite 221, Tucson, AZ 85719, and Dep. of Hydrology and Water Resources, BACKGROUND AND THEORY J.W. Harshbarger 122, 1133 East North Campus Drive, P.O. Box 210011, Univ. of Arizona, Tucson, AZ 85721; T.P.A. Ferré, Dep. of An electrical resistance measurement in a porous medium Hydrology and Water Resources, J.W. Harshbarger 122, 1133 East can be idealized as a measurement of three resistances in North Campus Drive, P.O. Box 210011, Univ. of Arizona, Tucson, series: the bulk electrical resistance of the medium, which AZ 85721; A.H. Christensen, U.S. Geological Survey, 5735 Kearny includes solid grains and pore water, (Rm), and a contact resisVilla Road, San Diego, CA 92123; J.P. Hoffmann, U.S. Geological Survey, 520 North Park Ave., Suite 221, Tucson, AZ 85719. Received 14 May 2002. *Corresponding author (kblasch@usgs.gov). Abbreviations: ER, electrical resistance [sensors]; PVC, polyvinyl chloride; TDR, time domain reflectometry. Published in Vadose Zone Journal 1:289–299 (2002).

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

Geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the San Gorgonio Pass region. This picture serves as the basis for inferring past fault movements within the San Andreas system, which are fundamental to understanding the tectonic evolution of the San Gorgonio Pass region. Interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of San Gorgonio Pass beneath basement rocks of the southeast San Bernardino Mountains. Subtle, long-wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of San Gorgonio Pass and south under Peninsular Ranges basement, and has a southern edge that is roughly parallel to, but 5–6 km south of, the surface trace of the Banning fault. This deep magnetic body is composed either of upper-plate rocks of San Gabriel Mountains basement or rocks of San Bernardino Mountains basement or both. We suggest that transpression across the San Gorgonio Pass region drove a wedge of Peninsular Ranges basement and its overlying sedimentary cover northward into the San Bernardino Mountains during the Neogene, offsetting the Banning fault at shallow depth. Average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data. Seismicity suggests a deeper detachment surface beneath the deep magnetic body. This interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross-sectional view. Given the multilayered nature of deformation, it is unlikely that the San Andreas fault will rupture cleanly through the complex structures in San Gorgonio Pass.

Preliminary hydrogeologic assessment near the boundary of the Antelope Valley and El Mirage Valley groundwater basins, California

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Aquifer geometry, lithology, and water levels in the Anza–Terwilliger area—2013, Riverside and San Diego Counties, California

The population of the Anza–Terwilliger area relies solely on groundwater pumped from the alluvial deposits and surrounding bedrock formations for water supply. The size, characteristics, and current conditions of the aquifer system in the Anza–Terwilliger area are poorly understood, however. In response to these concerns, the U.S. Geological Survey, in cooperation with the High Country Conservancy and Rancho California Water District, undertook a study to (1) improve mapping of groundwater basin geometry and lithology and (2) to resume groundwater-level monitoring last done during 2004–07 in the Anza–Terwilliger area. Inversion of gravity data, including new data collected for this study, was done to estimate the thickness of the alluvial deposits that form the Cahuilla and Terwilliger groundwater basins and to understand the geometry of the underlying basement complex. After processing of the gravity data, the thickness of the alluvial aquifer materials was modeled by using all available lithology, density, and geophysical data. The thickest alluvial deposits (greater than 500 feet) are in the northern part of the study area along the south side of the San Jacinto fault zone, in the southern part of the Cahuilla groundwater basin, and in the western part of the Terwilliger groundwater basin. Through most of the area of alluvial materials, the thickness of the alluvium estimated from gravity data is less than 400 feet. Analysis of more than 900 drillers’ logs indicated that in areas having relatively thick alluvium, particularly along the San Jacinto fault zone and in the Terwilliger Valley, the alluvium is predominantly composed of sands and gravels. Fine-textured sediments appeared to be discontinuous rather than forming laterally extensive, low-permeability layers. More than 500 drillers’ logs indicated only bedrock is present, indicating that the fractured bedrock is an important source of groundwater, primarily for domestic use, in the study area. The depths of the holes drilled into the bedrock indicated that fractures potentially supplying water to wells persist in the upper few hundred feet and that the permeable zone of the fractured bedrock extends to depths greater than weathered zones in the upper part of the basement complex. Water-level data were collected from 59 wells during fall 2013. These data indicated that hydraulic head did not vary substantially with well depth and that the measured water levels in bedrock and alluvium were similar. Large offsets in groundwater altitude across the San Jacinto fault zone indicated that the fault zone is a barrier to groundwater flow in the northeastern part of the Anza Valley. On the basis of data from 33 wells, water levels mostly declined between the fall of 2006 and the fall of 2013; the median decline was 5.1 feet during this period, for a median rate of decline of about 0.7 feet/year. Based on data from 40 wells, water-level changes between fall 2004 and fall 2013 were variable in magnitude and trend, but had a median decline of 2.4 feet and a median rate of decline of about 0.3 feet/ year. These differences in apparent rates of groundwater-level change highlight the value of ongoing water-level measurements to distinguish decadal, or longer term, trends in groundwater storage often associated with climatic variability and trends. Fifty-four long-term hydrographs indicated the sensitivity of groundwater levels to climatic conditions; they also showed a general decline in water levels across the study area since 1986 and, in some cases, dating back to the 1950s.

Estimating natural recharge in San Gorgonio Pass watersheds, California, 1913–2012

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