include such activities as the stabilization and jettying 

 of inlets, dredging of deep shipping channels in the bay 

 and through the barrier island system, and installation 

 of additional barrier features. Other factors include 

 commercial shell dredging activity, which has been 

 attributed for removing about 50% of the volume of 

 Nueces Bay during the period of 1950 to 1968 (Ward 

 1997), and the conversion of tidal marshes to upland, 

 particularly along the south shore of Nueces Bay, 

 which has transformed approximately 15% of the area 

 of Nueces Bay since 1925 (Ward 1997). 



Salinity and Freshwater Inflow 



After compiling and analyzing salinity data throughout 

 the Nueces Estuary from the 1950's through the 

 1990's, Ward and Armstrong (1997) found diat the 

 much of the estuary exhibited a well-defined increasing 

 trend in salinity, including Nueces Bay and most of the 

 open areas of Corpus Christi Bay. The average rates of 

 increase were not considered insigmficant. For 

 example, over ten years, the average rates of salinity 

 increases would result in an increase of average saHmty 

 of Corpus Christi Bay by 0.5 ppt and Nueces Bay by 

 2.5 ppt (Ward and Armstrong 1997). After inspection 

 of the data, these authors determined that the greatest 

 contributor to the declining trend was the reduced 

 frequency of occurrence of high-flow events in the 

 Nueces River. Several factors have been identified as 

 having possibly contributed to reduced stream flow in 

 the Nueces River, including an increase in water 

 storage and evaporation due to the construction of 

 reservoirs, an increase in the consumption of water 

 withdrawn from streams or shallow aquifers and a 

 possible decrease in rainfall within the greater 

 watershed (Asquith et al. 1997). 



Since 1935, three main-stem reservoirs have been 

 constructed on the Nueces River and its tributaries. 

 These dams include La Fruta Dam, Wesley Seale Dam 

 (Lake Corpus Christi), which replaced La Fruta Dam, 

 and Choke Canyon Dam (Figure 2-14). A direct result 

 of their construction was that the combined basin 

 storage capacity dramatically increased over a relatively 

 short period from 67,848 10' m' (55,000 acre- ft) in 

 1935 to over 1,221,264 10' m' (990,000 acre-ft) in 1982 

 (Figure 2-14). Wesley Seale Dam lies on the lower 



reach of the Nueces River, and Choke Canyon Dam 

 lies on the lower Frio River, the largest tributary to the 

 Nueces. 



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 

 171,470 10' m' (139,000 acre-ft) for mumcipal and 

 industrial use, and a portion of delivered water returns 

 to the estuary through treated return flows. Because of 

 the relative shallow depth of the two reservoirs and the 

 relatively hot 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 reser\^oir system 

 (Hilzinger 2000). 



Another factor possibly contributing to decreased 

 stream flow in the lower Nueces River include 

 increased non-reservoir surface water withdrawals in 

 the greater watershed. For example, long-term (1940 

 to 1990) analysis of reported surface water withdrawals 

 in the basin upstream of the reservoirs indicates an 

 increase of about 60 % from 1965 to 1990 (Green and 

 Slade 1995), which could have reduced the amount of 

 inflow to the reser\^oLrs. 



Finally, a decreasing precipitation trend in the Nueces 

 watershed would be expected to reduce streamflow. 

 Howerver, after analyzing rainfall data from four south 

 Texas gauges (Cotulla, Beeville, 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 2- 1 5a through 

 2-1 5c). Using a baseline that began during the late 

 1940's {e.g.. Figure 2-15d), 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 etal. 



Chapter Two ♦ 2-15 



