imposed by the river (as represented by mean super- 

 elevation) was considerable (greater than 1.21 m, or 

 4.0 ft) (Table 3-3). This indicates, unlike during Events 

 12 through 14, the diversion rate into Rincon Bayou 

 during Events 16 and 17 was predominandy a fiinction 

 of the elevated stage in the Nueces River. As each 

 principal peak receded, discharge through the overflow 

 channel dropped sharply (Figure 3-7). At the event's 

 conclusion, the loss of stage in the river immediately 

 resulted in a sharp decrease in the rate of discharge, 

 and the upper delta discharged a significant amount of 

 water back into the river. This pattern, the reversal of 

 diverted flow at the end of large river flow events, 

 would become common in subsequent events. During 

 both events, the Rincon Overflow Chaimel was 

 activated, diverting water into the extensive tidal flat 

 area of the upper delta. 



FaU1997: Event 18 



The fall event of 1997 (Event 18) resulted from very 

 heavy precipitation in the lower Nueces Watershed. 

 This event, although comparable to Event 16 in 

 regards to positive flow and maximum stage in Rincon 

 Bayou, was different from the previous two events in 

 one significant way. Unlike Events 16 and 17, which 

 were events responding to precipitation higher in the 

 basin. Event 1 8 was a hydrographic response to 

 intense local precipitation direcfly on the study area 

 and lower watershed. Approximately 23 cm 

 (9.17 inches) fell within the area during the first 6 days 

 of the event, contributing to heavy local runoff and 

 artificially elevated water levels in the upper delta 

 and Nueces Bay (as reflected in mean and peak stage). 

 As a result, discharge into the delta from the river 

 actually began before the river (gauged at Calallen) 

 began to respond to the event (Figure 3-7). 



As with the two previous events, the rate of diversion 

 into the delta fell off sharply as soon as stage in the 

 river began to drop, and, like Event 17, remained 

 negative for the concluding days of the event. 

 Although the total positive flow diverted into Rincon 

 Bayou during this event was about the same as that for 

 Event 16, over 60% of this volume flowed back into 

 the river at the end of the event. This difference was 

 attributable to the fact that water levels in the Nueces 



River at the point of diversion (and therefore, the 

 hydravilic head) were not maintained after the river 

 crested, as was the case with Event 16. 



FaU 1998: Events 21 through 27 



For purposes of event analysis, the fall of 1998 was a 

 very complicated period. The careful observer of 

 Table 3-3 and Figure 3-8 will recognize that, as with 

 Events 12 through 14, Events 21 through 27 were 

 essentially contiguous, or that the ending day of each 

 event immediately preceded the beginning day of the 

 next. The challenge in interpretation was separating 

 the effects of a bewildering number of influencing 

 factors. For example, during this 96-day period, two 

 tropical storms made landfall in the region, a fall- 

 maxima high water event occurred in the Gulf of 

 Mexico, over 56 cm (22 inches) of local rainfall was 

 recorded in the study area, over 219,820 10' m' 

 (178,194 acre- ft) flowed from Nueces River into the 

 bay and over 5,092 10' m' (4,128 acre- ft) was diverted 

 into the upper delta. For purposes of interpreting 

 biological responses in the delta, one may justifiably 

 consider each numbered event as a mere temporal 

 component of the greater autumn occurrence of 1998. 



Beginning on August 17, heavy rain episodes caused by 

 a cool air mass sagging into central Texas began to 

 occur over much of south Texas. Because the rainfall 

 followed record drought conditions between April and 

 July, very httie runoff resulted. In fact, over 15 cm 

 (6 inches) of precipitation feU over a 2,000-km" area in 

 the Frio River watershed and produced absolutely no 

 flooding (Patton 1998). The rainfall continued 

 throughthe day on August 1 8, with a wide-spread area 

 in the greater Nueces watershed receiving 20 to 33 cm 

 (8 to 13 inches). 



Tropical Storm Charley made landfall near Port 

 Aransas the night of August 21 (Events 21 and 22). 

 As the storm center moved slowly inland toward the 

 Hill Country west of San Antonio, it produced 5 to 

 8 cm (2 to 3 inches) of rain in a three-hour period, 

 with the heaviest rainfall resulting from "feeder" bands 

 that wrapped around the center. These bands were 

 moving very slowly and dropped several inches of rain 



Chapter Thne ♦ 3-17 



