overall salinity gradient. At any point in time, the 

 salinity is a result of the combined effects of the two 

 types of processes, but their relative importance 

 depends upon the characteristics of the freshet 

 response. 



In Nueces Delta 



In the Nueces Delta, variations in salinity 

 concentrations of the pools, channels and marshes are 

 driven by the same mechanisms as in the bays (i.e., 

 water-mass exchanges invohring both intrusion and 

 extrusion processes), with two sigtiificant differences. 

 First, because much of the water in the delta is 

 segregated into shallow pools and channels by higher 

 land formations, many of these become frequently 

 isolated from the bay due to water level fluctuations. 

 This seclusion magnifies the effect of evaporation, 

 resulting in the concentration of salts from bay water 

 into the soils and water of the delta. The opportunity 

 for continuous dilution by very large volumes of bay 

 water enjoyed elsewhere in the estuary is not readily 

 available in the delta. 



Second, the magnitude, frequency, duration and timing 

 of freshwater flow events (or freshets) into the Nueces 

 Delta are burdened by one additional condition not 

 applicable to Nueces Bay. This condition is that the 

 stage (water level) attained by the flood event in the 

 river must exceed the minimum flooding threshold of 

 the river in the delta segment of the stream 

 (determined by the elevation and dimensions of the 

 lowest portions of the river bank) (Figure 2-5). 

 Therefore, unlike Nueces Bay, the opportunity for 

 freshets in the delta are limited to discrete periods of 

 time when the river hydrograph sufficiendy meets this 

 condition. If the flooding threshold is not met, the 

 flow event in the river will bypass the delta, providing 

 Nueces Bay with an intrusion event without the same 

 courtesy for the delta. 



These periodic deltaic inundation events usually coin- 

 cide with tropical storm activity in early autumn or 

 witii the passage of frontal systems in late spring 

 (Texas Department of Water Resources 1982). Such 

 flooding events flush the numerous channels and 

 ponds of the delta, and inundate large areas to an 



Figure 2-5: Nueces River (looking downstream) just 

 below the IH 37 bridge under flood conditions (April 

 1992). During such high-flow events, the river spills fresh 

 water into the upper Nueces Delta from several low points 

 along the north (left) bank. 



Photo courtesy of the Bureau of Reclamation. 



extent and depth governed by the volume and duration 

 of die flood event (Ward 1985) (Figure 2-6). These 

 discrete events also serve as a mechanism whereby 

 salts, as well as organic materials and other nutrients 

 produced in the delta marsh, are exported from the 

 delta into the greater Nueces Estuar)'. Because of 



Figure 2-6: Natural flooding of the upper Nueces 

 Delta, April 1992. Such freshwater inundation events 

 significantly contribute to the biological productivity of the 

 delta by providing a medium for nutrient exchange and 

 biochemical cycling, supplying freshwater to marsh plant 

 communities, transporting detrital and other nutrient 

 materials from the established marsh vegetation and 

 sediments to the bay, and buffering bay salinity. 



Photo courtesy of the Bureau of Reclamation. 



Chapter Tm ♦ 2-7 



