Water level in Rincon Bayou (at least since the Nueces 

 Overflow Channel was opened) was the most 

 responsive parameter to hydro-meteorological forces in 

 the upper delta during the demonstration period 

 (Table 3-3). 



Summary of Selected Individual 



Events 



As can be observed from Figures 3-4 through 3-9, 

 most of the observed hydrographic events involving 

 large flows in the Nueces River occurred during the 

 latter portion of the demonstration period, particularly 

 during 1997, 1998 and 1999. This absence of such 

 events in the early part of the demonstration period 

 was attributable to the fact that, during the first few 

 years, south Texas experienced a moderate to severe 

 drought. However, although large freshwater diversion 

 events were absent during this period, modest flow 

 events into Rincon Bayou from the Nueces River did 

 occur {e.^.. Events 12, 13 and 14, and probably a few of 

 Events 6 through 10). The driving mechanism for 

 these events was not flow in the river but other hydro- 

 meteorological forces affecting water level in the 

 Nueces Estuary (Table 3-3). Several other similar 

 exchange events occurred throughout the 

 demonstration period. 



Fall 1996: Events 12 through 14 



One example of these kinds of small exchange events 

 were Events 12, 13 and 14, which resulted from a fall 

 maxima high water event in the Gulf of Mexico 

 (Table 3-3). From October 3 through 8, as the water 

 levels in Nueces Bay and Rincon Bayou increased, a 

 sustained positive flow occurred through die overflow 

 channel, which peaked on October 6 at 1.04 m'/s 

 (36.9 cfs) (Figure 3-10). During same period, only a 

 minimal amount of river flow (no more than 

 0.10 m^/s, or 3.5 cfs) passed over Calallen Diversion 

 Dam. However, during October 9 through 14, flow in 

 the river rose to about 4.67 m'/s (165 cfs) on 

 October 12, while water levels in the upper delta and 

 bay began to decrease. The surprising result was that 

 water diversion through the channel reversed direction 

 and flowed from the upper delta into the Nueces River 



until October 15, when the water level in the bay again 

 began to increase. This behavior, which continued 

 through Events 13 and 14, demonstrated that, at low 

 flow volumes in the Nueces River, diversions through 

 the overflow channel were driven primarily by water 

 level variations in Nueces Bay and the upper delta. 



The USGS made several salinity measurements at the 

 Rincon gauge during Event 12. Salinity was measured 

 on October 4, 5 and 6, which was the period of 

 sustained positive flow through the overflow channel. 

 The salinity values for each day were 2.0, 3.9 and 

 7.2 practical saHnity units (psu) (which are approxi- 

 mately equivalent to parts per thousand), respectively. 

 During this period, no flow occurred in the Nueces 

 River on October 4 and 6, and only 0.10 m'/s (3.5 cfs) 

 occurred on October 5. The increasing salinity values 

 over this 3-day period indicates that flow was moving 

 up the Nueces River channel and into Rincon Bayou as 

 a result of the rising water level in Nueces Bay. 

 Therefore, the total net diversion into Rincon Bayou 

 during these three events (289 10' m', or 234 acre-ft) 

 was relatively fresh water. 



Summer 1997: Events 16 and 17 



The first significant occurrence of freshwater flow 

 during the demonstration period occurred from June 

 21 through July 27, 1997 (Events 16 and 17). These 

 two events occurred one immediately after the other, 

 and were derived from the same basin-wide 

 precipitation event. This storm was one of the many 

 tropical/middle latitude heavy rain events common to 

 south Texas. On June 21, a near-stationary low 

 pressure system over south-central Texas began to 

 move east and north, causing scattered showers and 

 thundershowers over a large part of north Texas, the 

 Texas Hill Country and central Texas. This movement 

 allowed tropical moisture to move in from the south 

 and feed into the area of instability, lift and daytime 

 heating in the afternoon, which resulted in a second 

 round of locally heavy rain in the greater Nueces 

 watershed. 



Rainfall amounts with this second rain event varied 

 from 23 to 58 centimeters (cm) (9 to 23 inches) over 

 the Texas Hill Country, and between 13 and 25 cm 



Chapter Thne ♦ 3-9 



