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Analysis of discharge hydrographs for streamflow-gaging stations on the Nueces River at Cotulla, Tilden, and Simmons indicate that significant water losses occur along the 108-mile reach from Cotulla to Simmons during storm- runoff. Computed losses along the 83-mile reach from Cotulla to Tilden for 15 storm periods range from 32 to 59 percent of the total runoff volume passing the Cotulla gage. For six storm periods that occurred while the gage at Simmons was in operation, computed losses from Cotulla to Simmons averaged 48 percent of the storm runoff passing the Cotulla gage. Estimates of total-annual losses were made with the aid of a regression model developed to relate monthly rainfall totals to monthly runoff. The model was calibrated using runoff data for San Casimiro Creek, the only gaged tribu- tary in the study area, and monthly rainfall totals from nearby rain gages. The calibrated model was used with monthly rainfall totals from six National Weather Service rain gages in or near the study area to estimate monthly-runoff volumes for the ungaged area between Cotulla and Simmons. Total annual water losses, estimated with the aid of the regression model, ranged from 46,600 acre- feet during water years 1969 to 368,500 acre-feet during water year 1967, and averaged about 174,000 acre-feet for water years 1966-77. INTRODUCTION The Texas Water Plan, a guide for development of the State's water resources, indicates that the lower Nueces River basin will be a water- deficient area in the near future. The coastal areas of the lower Nueces River basin, which include the greater Corpus Christi area, are experiencing rapid urban and industrial growth. Increasing water supplies will be needed to support the economy required to fulfill the needs of that area's increasing population. Currently, these coastal areas get most of their water supplies from the surface-water resources in the Nueces River basin. Upstream, principally in the Winter Garden area near the middle of the Nueces River basin, more than 500,000 acres of fertile land is suitable for irrigation. During 1974, about 300,000 acres were being irrigated, mostly from ground-water resources. However, existing ground-water resources are being depleted, and available surface-water resources are insufficient to sustain even the 1974 irrigation. Suggested solutions by State and local officials to the existing and anticipated water supplies and related problems of the Nueces River basin include importing large quantities of water from outside the basin along with optimum development and wise use of existing water supplies within the basin. To accomplish the latter, the U.S. Burea the Interior, has undertaken a study to identify available water resources of the Nueces River in cooperation with the Bureau of Reclamation of Reclamation, Department of the long-term water needs and basin. This report was prepared as part of their overall study. PURPOSE AND SCOPE OF THIS REPORT The purpose of this study is to provide data and hydrologic interpreta- tions for use by the Bureau of Reclamation in evaluating and quantifying natural water losses that are known to occur along softie reaches of the Nueces River. Specifically, this report presents data and analyses describing the convey- ance characteristics of that reach of the Nueces River from Cotulla to Simmons. A proposed reservoir on the Nueces River at Cotulla would be located at the upstream end of the study reach. The efficiency of the river channel for transporting reservoir releases through the study reach to downstream water- users may be a prime factor in determining the feasibility of constructing the reservoir. To aid the Bureau of Reclamation in determining the feasibility of this project, the U.S. Geological Survey determined the efficiency of the river channel for transfer of water through the reach. The magnitude of the losses expected to occur during periods of continued reservoir release were estimated. DESCRIPTION OF THt BASIN The Nueces River basin is located in south-central Texas and extends from the deeply dissected limestone plains of the Edwards Plateau in Edwards County across the Balcones Fault zone and the Gulf in the vicinity of Corpus Christi (fig. 1). and east by the Colorado and Guadalupe River coastal basin, and on the Nueces-Rio Grande coastal astal Plains to the Gulf of Mexico he basin is bounded on the north asins and the San Antonio-Nueces west and south by the Rio Grande basin and the basin. Total basin drainage area is about 16,950 square miles. Principal streams in the Nueces River basin include the Atascosa and Frio Rivers, which enter the Nueces River upstream from Lake Corpus Christi but downstream from the study area. The headwaters of the Nueces River originate at an elevation of about 1,220 feet in Edwards County. The river crosses the Balcones Fault Zone down- stream from Laguna and flows generally southeasterly through the coastal plain into Nueces Bay, an arm of Corpus Christi Bay