effect on the use of the data in the biological analyses. However, no super-elevation can be computed when 

 one of the two stage values is missing, so these peg values become gaps in the super-elevation data stream. 



Comments on the Data 



upon starting the display at the beginning of the data period, vi^ 1 992, one is immediately struck by the 

 substantial flow hydrographs in the Nueces River. A series of large hydrographs begin in February and 

 continue through June, holding salinity concentrations to virtually fresh throughout this period and the early 

 summer. The Interim Order mandated releases begin in September 1992, and are manifested as the small 

 pvdses of inflow occurring near the end of each month. The 1992 fall high water occtirs in early October. 

 During this period, the lunar declination is near its maximum attainable value, and only one front of any 

 consequence occurs in the fall, the remainder of the period being dominated by onshore flow from the Gulf. 



In late June 1993, a substantial rainfall event over a three day period occurred. Simultaneously, a spike in water 

 levels of over 0.5 m is registered synchronously in Nueces and Corpus Christi Bays. The cumulative rainfall in 

 the event totaled 0.23 m as measured at Corpus Christi. Evidentiy, such a large volume of rainfall can account 

 for at least part of the observed excursion in water level. A few days later the associated hydrograph on the 

 Nueces reached Calallen and delivered a cumulative 3.3 Mm^ of inflow. Spread over the 500 km" of combined 

 surface area of Corpus Christi and Nueces Bays, this would amount to less than a cm of additional water depth. 

 Indeed, the water-level data from July show no discernible response to the Nueces inflow. 



The moral of this comparison is that hydrograph events on the river could be expected to have littie impact on 

 the elevation of water in the bay, whereas sudden diluvial rainstorms may have, if the rainfall area encompasses 

 a substantial portion of the bay area. Several such spikes can be seen in the water level histories that 

 correspond to intense rainfalls. On the other hand, the Calallen flow hydrographs create a greater response in 

 the salinity than a rainfall event. The former in fact is a water-mass displacement process, while the latter is a 

 dilution of the rainfall depth throughout the water depth. 



The summer 1993 water-level history is a good example of the summer seasonal low water, due to the absence 

 of other hydrographic factors from late June through August. In this same year, the subsequent fall high water 

 is a rather minim al event. The 1 994 fall high water is more typical of this annual event 



The only significant hydrographic events occurring during the period after opening of the Nueces Overflow 

 Channel but before the operation of the USGS Rincon gauge are foimd in October 1995, the month during 

 which the channel was opened. A low but fairly steady flow over Calallen occurred during October but abated 

 the day before the channel was opened. Iribeck observed that the level of water in the Nueces did not acquire 

 the threshold to force flow through the overflow channel. On 28 October, two days after the channel was 

 opened, an intense rainfall event (over 20 cm in one day) created sufficient local flow to scour down die 

 channel (see Chapter 3 in the draft report). Although a spike ki bay water level occurs, the effect on salinity is 

 negligible. For the next several months, only a few minor rainfall events appear in the record, and the bay 

 salinity climbs, nearly monotonically, into the hyper-saUne range. 



Finally, it should be noted that there are other pathways for flow to enter Rincon marsh, vi2. the series of low 

 points in the north levee of the Nueces River. Only when stage in the rivet becomes sufficiendy high does flow 

 begin to pass these other openings. Based upon HEC-2 hydraulic model runs (Bureau of Reclamation 2000), a 

 relation has been developed between the flow in the Rincon channel and the total flow entering Rincon marsh 

 by aU of the available routes. The results of this modeling analysis are summarized in the following figure 

 showing the proportion of total flow into the marsh represented by the Rincon channel. For small river stages, 

 this is clearly 100% (Figure 2). Then the proportion rolls off in a sigmoid-like shape approaching a level of 

 about 5%. Considering the sources of error in aU of this, the total flow into Rincon marsh can be 

 approximately related to that in the Rincon channel as follows: 



Rincon Q Rincon Q < 200 cfs 



Combined Q (cfs) = 0.8 (Rincon Q - 200) Rincon Q 200 < Rincon Q < 450 cfs 



20 (Rincon Q) Rincon Q > 450 cfs 



Appendix B <* B-5 



