Increasing salinity has been a growing concern for users of waters from the Pecos River and the reservoirs it feeds in the Texas portion of the River's watershed. Irrigation water diverted from the river in the northern reach of this watershed is often only suitable for a limited number of crops, reducing harvesting options for local farmers. In the south, the Pecos feeds into the International Amistad Reservoir along the border with Mexico. During the 1990s, total dissolved solids concentrations in the reservoir rose as much as 10 mg/L per year and often approached the drinking water standard for potable water (1000 mg/L). Since this time, control efforts have focused on reducing the river's salinity, requiring the identification of salt sources. Hydrologic trend analysis and geochemical identification methods were used to determine these sources for the reach of the river between Red Bluff Reservoir and Brotherton Ranch. Between Red Bluff Reservoir and Coyanosa, flow diversions remove much of the flow that carries the salts, resulting in decreased salt loads, but also making the river more sensitive to evapotranspirative concentration. This sensitivity is evident in the river between Coyanosa and Girvin, where salinity begins to increase to the highest levels within the study area. However, salt loads increase here as well, indicating external salt sources as a contributor. The most substantial increase in bromide ions and the Br-/Cl- ratio appears between Grandfalls and Imperial, although no conclusion could be drawn regarding the identity of the source. The ratio continues to increase up to Girvin, where it appears that evapotranspirative concentration again has a significant effect. Here, several points drifted to the right of the groundwater mixing zones, plotting at values that were uncharacteristic of these sources.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Samuel M. Wilson. Vita. Includes bibliographical references.

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This thesis examines six prominent Pecos River Style rock art anthropomorph attributes to determine if they are found in limited geographic districts of the Lower Pecos Region. Both Boyd (2003) and Turpin (2004) have suggested that spatially-segregated motif distributions exist in the rock art and that these patterns are important in understanding regional prehistoric hunter-gatherer lifeways during the Archaic Period. This study verifies that the feather hip cluster motif is geographically limited, identified only in the neighboring Seminole and Painted Canyon systems. As part of this spatial analysis, the previously undocumented principle of intersite stylistic traditions is introduced. Possible explanations for these anthropomorph attributes are also discussed. Finally, structural analyses of the six attributes are presented.

Tens of millions of dollars have been spent to control Tamarix (saltcedar) trees along waterways in the Southwestern United States for the purpose of increasing streamflow yet no increase in streamflow has been demonstrated. The Pecos River Ecosystem Project (PREP) served as a case study to characterize surface and groundwater interaction along the Pecos River in Texas, assess the influence of saltcedar transpiration on stream stage and water table fluctuations, and evaluate the impacts of large-scale saltcedar control on baseflows. This is the first study that has investigated the influence of saltcedar transpiration on surface and groundwater interaction and the first to provide a mechanistic explanation for the lack of measurable increase in streamflow. Neither saltcedar transpiration nor saltcedar removal influenced hydraulic gradients, streambank seepage, or stream elevations. The results of the plot scale studies indicate saltcedar transpiration along the Pecos River is lower than reported elsewhere and therefore may not yield detectable increases in baseflow. To extend the study to a much larger scale, we analyzed annual baseflows at the downstream end of 340 km river reach from 1999 (pretreatment) through 2009. Surprisingly, baseflows declined for four years after the project began despite additional acreages of saltcedar treatment each year. However, baseflow surged in 2005 and remained higher than the pretreatment year (1999) through 2009. Additional detailed analyses of reservoir release and delivery records and rainfall are needed to better understand contributions of rainfall and flow regulation to this increase. Tracer based studies to determine the relative contributions of releases and groundwater would also enable a better interpretation of the change in baseflows. We did not investigate any other reported benefits, such as restoration of native plant species, or reduced soil salinity, of saltcedar control.

This thesis examines six prominent Pecos River Style rock art anthropomorph attributes to determine if they are found in limited geographic districts of the Lower Pecos Region. Both Boyd (2003) and Turpin (2004) have suggested that spatially-segregated motif distributions exist in the rock art and that these patterns are important in understanding regional prehistoric hunter-gatherer lifeways during the Archaic Period. This study verifies that the feather hip cluster motif is geographically limited, identified only in the neighboring Seminole and Painted Canyon systems. As part of this spatial analysis, the previously undocumented principle of intersite stylistic traditions is introduced. Possible explanations for these anthropomorph attributes are also discussed. Finally, structural analyses of the six attributes are presented.

Pre-existing structural elements can have substantial effects on fracture and fault development in younger strata, especially in areas that undergo significant changes in tectonic setting due to reactivation along older structures. This may affect reservoir permeability, yet remain difficult to detect in subsurface data. The focus of this study centers on two styles of pre-existing structures—Paleozoic thrust belts and Late Triassic rift faults in the Devils River Uplift and Maverick Basin, respectively—which affect the development of faults and fractures in Cretaceous strata. Fault and fracture data were characterized in both the outcrop and within a 3D seismic volume. Furthermore, the role of mechanical stratigraphy on fault and fracture style in both localities was examined. The Pecos River Canyon overlies the Paleozoic Ouachita fold-thrust belt with associated EW and SE-NW trending structures. At the surface, faults are expressed in two predominant orientations (N38E and N70E), which may be predictable angles if the pre-existing structures are reactivated by left lateral oblique slip. Detailed investigation of the fracture development related to these faults was conducted in a dry side canyon along the Pecos River. Mechanical layers were identified and mapped in outcrop to highlight fracture intensity variations between the different layers. The porosity and/or the degree of dolomitization are identified as controls on fracture development, with the lowest strength layer and least fractured being highly dolomitized with the largest porosity of any observed layer in outcrop. Southeast of Lewis Canyon, a 3D seismic of the Maverick Basin reveals linear discontinuities, interpreted as low-offset faults, within the Cretaceous Glen Rose through Austin Chalk that appear similar to those observed in outcrop along the Lower Pecos River. These faults are shown to have an increase in intensity within strata above older Late Triassic-age rift faults. It is proposed that the small faults form during reactivation of the rift faults and exhibit differential degrees of intensity and vertical terminations against six identified mechanical boundaries observed within the 3D seismic volume. / text

The segment of the Pecos River that extends from Red Bluff Reservoir until it discharges to the Rio Grande/Bravo near Langtry was studied in this project. Hydrologic behavior of the basin was analyzed between 1981 and 2000, the first ten year period for calibration and the second ten year period for validation by using Water Evaluation and Planning Software (WEAP, SEI, 2006). Simulated streamflows were compared with naturalized streamflows (RJBCO, 2003) at two control points, one in the middle of the basin near Girvin and the other one is at the end of the basin near Langtry. The purpose of the project is to create a valid model for water availability simulations in the Pecos River Basin to be used for future water availability simulations considering climate change effects. The basin was divided into two parts in order to evaluate the results, the upper basin and the entire basin (below Red Bluff reservoir) according to the location of control gages. Simulated streamflows closely match the naturalized flows at the Girvin station in the upper basin. Although the results at the Langtry station for the entire basin are not as good as Girvin, the model still reproduces streamflows well enough to represent the hydrologic behavior of the basin, especially for the base flow. Considering the complex geological structure of the Pecos River Basin below Red Bluff Reservoir, the results can be considered satisfactory. The model can be used for future water availability predictions in the basin considering climate change effects. / text