of the intake. Then in 1898, a pennanent rock-filled dam with a crest elevation of 0.46 m (1.5 ft) msl was 

 completed at the same location. In 1916, the first treatment plant was added at the Calallen pump station. In 

 1935, the current concrete structure known as Calallen Diversion Dam was completed with a crest elevation of 

 approximately 1.36 m (4.52 ft) msl. 



During 1917, another drought threatened the municipal water supply of the growing City of Corpus Christi and 

 the surrounding area. A series of engineering surveys identified a suitable site for a large reservoir on the 

 Nueces River some 35 miles upstream of Calallen Dam. The purpose of the new dam would be to store water 

 for release during the dry periods. Mathis Dam was originally constructed by the Cit}' of Corpus Christi early in 

 1930, but failed in November of that same year. In 1934, a second dam across the Nueces River (La Fruta 

 Dam) was constructed. This new reservoir had a storage capacity of approximately 67,849 10"* m' 

 (55,000 acre-ft). 



From 1930 to 1950, the population of the Corpus Christi area increased by over 400% (Corpus Christi 1990). 

 As a result of this increase in demand for municipal and industrial water, and from the loss of reservoir storage 

 in La Fruta Reservoir due to siltation, a new water source was sought, again from the Nueces River. After 

 several years of study, the State of Texas financed the design and construction of Wesley Seale Dam (Lake 

 Corpus Christi), which was completed in 1958. This dam was located about 300 m downstream from La Fruta 

 dam site and was some 6 m higher, inundating the former structure. During the first six years of operation. 

 Lake Corpus Christi was maintained at 26.8 m (88.0 ft) msl, or a capacity of 229,355 10^ m' (185,922 acre-ft), to 

 allow for depletion of oil fields located in the reservoir basin. The crest gates were finally closed on July 1, 

 1964, bringing the operational lake elevation to 28.6 m (94.0 ft) msl, thereby increasing the storage capacity to 

 372,550 10' m' (302,000 acre-ft). 



Forecasts of future water requirements for the Coastal Bend area indicated that demand would exceed the firm 

 aimual yield of Lake Corpus Christi during the 1 980's. In response to the Area Development Water 

 Subcommittee's recommendation, the City of Corpus Christi engaged the Bureau of Reclamation to sur\'ey the 

 lower Nueces River Basin to determine a feasible location for a new reservoir to supplement Lake Corpus 

 Christi. Reclamation began construction on Choke Canyon Dam in the summer of 1979, and the project was 

 declared "substantially complete" on May 18, 1982. Due to a drought affecting the Frio River's 14,323 km" 

 watershed, flow into the new reservoir was minimal during the first three years, and by May 31, 1987, the 

 reservoir was only at 48% of capacity. However, record rainfall on the Frio River watershed filled the reservoir 

 to 100% on June 18, 1987, and water was released as flood control for the first time. At this level. Choke 

 Canyon Dam impounds approximately 852,584 10' m' (691,130 acre-ft). 



Developed for the purposes of providing a reliable and municipal water supply and flood protection, these 

 dams have contributed to reduced streamflow in the lower Nueces River by their diminutive influence on larger 

 river hydrographs, and through direct water loss to consumptive uses and evaporation. The present permitted 

 firm yield of the reservoir system is 139,000 acre-ft, and a portion of the dehvered water returns to the estuary 

 through treated return flows. Because of the relative shallow depth of the two reser^'oirs and the hot summer 

 climate, evaporation from these two water bodies can remove a significant amount of water from the river 

 system. For example, during 1999 alone, over 217,730 10' m' (176,500 acre-ft) were lost to evaporation from 

 the combined reservoir system (Hilzinger 2000). 



Other Changes in the Nueces Watershed 



Another possible factor contributing to decreased stream flow in the lower Nueces River are increased non- 

 reservoir surface water withdrawals in the greater watershed. For example, long-term (1940 to 1990) analysis of 

 reported surface water withdrawls m the basin upstream of the reservoirs indicates an increase of about 60% 

 from 1965 to 1990 (Greene and Slade 1995), which includes much of the operational time period of the current 

 reservoir system. 



Also, a decreasing precipitation trend in the Nueces River watershed would be expected to reduce streamflow. 

 However, after analyzing rainfall data from four south Texas gauges (Cotulla, BeeviUe, Sabinal and Corpus 

 Christi) reflecting conditions for the Nueces watershed, Medina (2000) found that annual precipitation (using a 

 base period that consisted of data since 1900) produced no particular trend (Figures 2a through 2c). Using a 

 baseline that began during the late 1940's (e.g.. Figure 2d), annual precipitation portrayed an increasing trend 

 (Medina 2000). The most prominent and common feature of the precipitation data at all stations was the 

 drought of the late 1940's and early 1950's. Similarly, Asquith et a/. (1997) also found Utde evidence for 

 statistical trends in precipitation along the Coastal Bend from 1968 through 1993. 



C-4 ^ Analysis of the Historic Flow B^ffme of the Nueces River into the Upper Nueces De/ta 



