Inflow resulting from the record-setting flood event in 

 the Nueces River caused a substantial rise in water 

 levels in Nueces and Corpus Christi bays of up to 

 0.6 m above that of the Gulf of Mexico (Grozier et al. 

 1968; Corps of Engineers 1968). Grozier et al. (1968) 

 went on to report that water levels in the bay were slow 

 to fall (it took several days) because of the enormous 

 amount of runoff from the rains inland, even though 

 Corpus Christi Pass and two other new channels were 

 opened between the bays and the Gulf by the storm. 



Salinity and Freshwater Inflow 



In general, salinity concentrations in estuary systems 

 are stratified horizontally and vertically, both of which 

 are primarily determined by the size and location of the 

 freshwater source(s). Over the area of the estuary 

 (horizontal), the gradient of fresh to brackish to marine 

 salinity concentrations (which is to 1 5 to 30 parts per 

 thousand (ppt) salt, respectively) progresses from the 

 areas closest to the freshwater source {i.e., river's delta 

 and mouth), to the secondary bay(s), then to the 

 primary' bay. The degree of salinity stratification in the 

 water column (vertical) depends upon the intensity of 

 mixing. 



In Nueces Estuary 



As in most estuaries, there are substantial spatial 

 gradients in salinity across the Nueces Estuary, not 

 only because of the great range in hydro-chmatology, 

 but also because of the location of the river drainages 

 and the variable influence of the sea (Ward and 

 Armstrong 1997). Based on evaluation of long-term 

 data, seasonal variations in the salimty of the Nueces 

 Estuary, other than a proclivity for sUghdy higher 

 salinity concentrations in the summer, were not readily 

 evident (Vt'ard 1997). Salinity variations within the 

 water column {i.e., stratification) in the Nueces Estuary 

 was found to be minimal (less than 0.5 ppt), but the 

 largest values of this (small) gradient typically occurred 

 in Nueces Bay, nearer to the freshwater source (Ward 

 1997). One exception to this generalization is 

 evaporation stratification, which t)'pically occurs in 

 stagnant portions of Corpus Christi Bay during the 

 summer. In this case, the salinity gradient in the water 



column can be as much as 6 ppt and often associated 

 with hypoxia (Ritter and Montagna 1999). 



Short-term vacillations of salimt)^ values within the 

 greater Nueces Estuary are primarily in response to 

 water-mass changes. One of the most common and 

 important contributors to this vacillation is the 

 response of salinit)' to an influx of freshwater 

 (extrusion) and the subsequent recover)^ of salinit)- as 

 the inflow event diminishes (intrusion). These two 

 mechanisms involve different physical processes, and 

 therefore are not symmetnc events. Extrusion is 

 effected by a replacement of water volume due to the 

 rapid influx of the inflow (river) hydrography, 

 commonly called a "freshet" event. Intrusion is 

 accomplished by the internal circulations gradually 

 returning higher salimty water to the upper reaches of 

 the bay and delta. Extrusion typically occurs quickly, 

 within a few days to several weeks (depending upon 

 the region and the size of the freshet event), while 

 intrusion typically requires weeks to months. Because 

 the synoptic events producing the runoff are also 

 frequendy accompanied by a frontal passage and 

 regional rainfall, displacement of salt water by fresh is 

 assisted by both wind forcing (frontal cross-bay 

 transports and efflux to the Gulf) and surface 

 precipitation surfeit. Intrusion, occurring over a longer 

 time frame, is assisted by the t}'pical evaporation deficit 

 at the surface. 



One factor of the regional hydro-chmatolog}' that 

 facilitates identification of these kinds of events is that 

 the largest freshets (which therefore have the potential 

 for the greatest salinity response) are very widely 

 spaced in time. Occasionally several such freshets 

 occur closely enough in time that intrusion from one is 

 superposed on extrusion from the other, making 

 interpretation of the salinity response quite 

 complicated. From a physical %'iewpoint, extrusion is 

 an advective process, invoh'ing the wholesale 

 displacement of water from the upper bay to the lower, 

 or (for extreme events) into Corpus Christi Bay or 

 even the Gulf of Mexico. Intrusion, on the other 

 hand, is the combined effect of smaller water mass 

 transfers, such as tidal exchanges and internal 

 transports, whose cumiolative behavior is more of a 

 diffusive process, operating to mix out and reduce the 



2-6 ♦ Study Ana 



