Exchange Events 



Exchange events were considered to be frequent, low- 

 volume interactions between the channels and pools of 

 Rincon Bayou and either adjacent water body (Nueces 

 Bay or Nueces River) (Figure 3-17). Exchange events 

 were primarily caused by daily differences in water level 

 elevations, although these differences were in turn the 

 result of a variety of other factors like tide, wind, river 

 inflow, etc. The net flow volume for these types of 

 events was generally low (less than 123 10' m', or 

 100 acre- ft), and could be either positive or negative 

 through the Nueces Overflow Channel. Although the 

 effects of exchange events were confined to the 

 channels of Rincon Bayou, they provided considerable 

 dilution and mixing of ambient waters, especially in the 

 upper delta. Events 11, 15, 22 and 31 were typical 

 examples of exchange events (Table 3-3). Prior to the 

 demonstration project, the Rincon Bayou and the 

 upper delta were completely isolated from such daQy 

 interactions with the river. 



Positive-Flow Events 



Figure 3-17: Typicai v:i.. of the Nueces Overflow 

 Channel during tidal exchange. The view is looking 

 downstream (east) from the Nueces River. 



Photos courtesy of the Bureau of Reclamation. 



the demonstration period, the amount of fresh water 

 diverted into the upper Nueces Delta was increased by 

 about 732%, and most of this increase was attributable 

 to positive-flow events. 



The second event type, positive-flow events, were 

 considered to be infrequent, large-volume events which 

 resulted in a positive flow of water from the Nueces 

 River into Rincon Bayou (Figure 3-18). Because these 

 events were primarily driven by flow events in the 

 Nueces River, they typically occurred during the spring 

 or faU. Unlike exchange events, the volumes associated 

 with positive-flow events (usually greater than 

 123 10' m', or 100 acre-ft) did not simply dilute water 

 in Rincon Bayou but also displaced it to a considerable 

 extent. Because of their magnitude, positive-flow 

 events were not confined to Rincon Bayou but 

 frequendy affected the lower adjacent flats, channels 

 and pools. Depending upon their magnitude, such 

 events might or might not have also inundated the 

 higher marshes and tidal flats of the delta. As 

 previously discussed, the actual diverted volume and 

 effectual extent of any one such event was greatiy 

 dependant upon the ambient water level in Nueces 

 Bay. Events 26, 27, 29 and 34 were typical 

 examples of positive-flow events that did not inxmdate 

 higher adjacent marshes and flats (Table 3-3). During 



Figure 3-18: View of the upper Rincon Bayou 

 (background) during a typical positive-flow event 

 (Event 16). The head-water channel of Rincon Bayou 

 (about 300 m downstream of the Nueces Overflow 

 Channel) is in the foreground, and the western-most 

 MoPac Railroad bridge is center. Upper Rincon Bayou is 

 in the background. The photograph was taken on 

 June 26, 1997. 



Photo courtesy of the Bureau of Reclamation. 



ChapterThree ♦ 3-23 



