not have fairly depicted the range of water level 

 excursion during an event. Raw daily stage data from 

 the Rincon gauge, which is reported in local time, was 

 corrected for GMT {i.e., the 15-minute data were re- 

 averaged) to allow temporal consistency with the other 

 data variables. The super-elevation of water levels, 

 determined by subtracting the Nueces Bay stage from 

 the Rincon Bayou value, was used to determine the 

 predominant influence on stage in upper Rincon 

 Bayou. On an instantaneous basis, the super-elevation 

 is the direct force for discharge through the Nueces 

 Overflow Channel. 



Salinity — The salinity response of upper Nueces Bay 

 to an inflow event would be expected to be an initial 

 drop in concentration, followed by a slow increase with 

 time, or "recovery". However, this approach of 

 integrating the salinity parameter could possibly 

 understate the response of salinity to lower magnitude 

 hydrographic events, because the entire response could 

 be occur within the event's duration and therefore 

 elude detection. Foxir separate indicators were thus 

 computed from the salinity data for each hydrographic 

 event: 1) the salinity value at the beginning of the event 

 (start), 2) its net incremental change at the close of the 

 event (chg), 3) the percent of the beginning value that 

 the increment represented (% chg), and 4) the 

 minimum value attained during the event (min). As so 

 defined, the sign of the incremental (percent) change 

 indicates whether salinity increased or decreased over 

 the duration of the event (negative for a decrease and 

 positive for an increase). 



RESULTS 



In this analysis, hydrographic events were delineated 

 according to the mechanism or feature exhibiting a 

 response: namely, the response of flow in the Nueces 

 River or in Rincon Bayou; water level in Nueces Bay or 

 in Rincon Bayou; and salinity in upper Nueces Bay. A 

 oirsory inspection of the hydrographic events 

 presented in Table 3-3 reveals the fact that each of 

 these response mechanisms can occur in isolation, 

 without the involvement of the others. This indicates 

 that a certain degree of care must be used in 

 determining how a hydrographic event has influenced 



the chemistry and ecology of the project area. For 

 example, if inundation and de-watering were of 

 concern, then the extent to which events accomplished 

 a response of water level would be of central interest, 

 and the most important of these was the response of 

 water level in Nueces Bay for the lower delta and in 

 Rincon Bayou for the upper delta. If freshwater inflow 

 to Nueces Bay from the river were the major 

 determinant, perhaps through a sediment or chemical 

 load, then the flow events in the Nueces River would 

 be regarded as most important. If it were freshwater 

 flow into the upper delta, then flow in Rincon Bayou 

 would be critical. Finally, if the depression of salinity 

 were a key interest, then the salinity events in Nueces 

 Bay would be of primary concern. 



Overview of Hydrographic Events 

 That Occurred During the 

 Demonstration Period 



For the period investigated {i.e., October 1994 through 

 December 1999), a total of 37 hydrographic events 

 were identified (Table 3-3). Five of these events 

 occurred prior to the opening of the Nueces Overflow 

 Channel, and thirty-two afterward. These events were 

 highly variable in the magnitude of their responses, 

 durations and in the subset of hydrographic variables 

 in which a response occurred. Some were associated 

 with seasonal high waters in Nueces Bay, some were 

 salinity responses elicited only by internal circulations 

 of the bay, some were responses to intense rainfall and, 

 of course, some were in fact inflow hydrographs in the 

 Nueces River. Most (28) of these events occurred after 

 operation of the USGS Rincon gauge in the Nueces 

 Overflow Channel, allowing the direct measurement of 

 flow diverted by the demonstration project, the 

 associated water level rise and in situ precipitation. 



Of the total 37 events observed, 1 5 met the flow 

 criteria for a flow event in the Nueces River, 16 met 

 the criteria for a stage event in Nueces Bay and 21 

 met the criteria for a salinity event. Of the 28 events 

 occurring after the installation of the Rincon gauge, 

 20 met the criteria for a flow event in Rincon Bayou, 

 27 met the criteria for a stage event in Rjncon Bayou 

 and 14 met the criteria for a super-elevation event. 



Chapter Jhne ♦ 3-7 



