APPENDIX C 



Analysis of the Historic Flow Regime 

 of the Nueces River into the Upper 

 Nueces Delta and of the Potential 

 Restoration Value of the Rincon Bayou 

 Demonstration Project 



Michael J. Irlbeck U.S. Bixreau of Reclamation, Austin 



Dt. George H. Ward Center for Research in Water Resources, University of Texas, Austin 



In preparation for publication. 



INTRODUCTION 



Deltaic ecosystems are typically supported by at least one principal river system, and therefore have adapted to 

 and are dependant upon the dynamic fluctuations of that river's flow pattern. This natural flow regime includes 

 the fuU range of a river's flow quantity, timing and variability, which may fluctuate by hours, days, seasons, years 

 and even decades. Until recendy, the importance of the natural stream flow variabUity in maintaining healthy 

 aquatic ecosystems has been underappreciated in a water development and management framework (Kart 

 1991, Voiiet al. 1997). However, it has been shown that the integrity of flowing water systems depends largely 

 upon the components of their natural dynamic character, which is critical in regulating a variety of ecological 

 processes within these ecosystems (Poff and Ward 1989, Richter et al. 1996, Walker et al. 1995). 



In south Texas, the ecological integrity of the Nueces Delta has been primarily determined by the natural flow 

 regime of the Nueces River, which includes the magnitude, frequency, duration and timing of flow events in the river. 

 Occasionally, high flows in the Nueces River cause the water elevation to exceed the diversion threshold (or, 

 rise above the lowest point along the river bank) and spill into the upper delta. These periodic freshwater 

 inundation events drive several key biological processes important for estuary productivity (Longley 1994). The 

 magnitude of a given flow event is simply the amount of water moving past a fixed location per uiut time. 

 Event duration is the period of time associated with a specific flow condition. The frequency of an event refers 

 to how often a flow at a given magnitude recurs over some specified period of time. And the timing of events, 

 or their seasonal predictabiUty, refers to the regularity with which flow events of a specified magnitude occur 

 within a specified time scale. 



In 1993, as part of a broader initiative to restore freshwater flows to the greater Nueces Estuar)', the United 

 States Bureau of Reclamation (Reclamation) began a multi-year demonstration project designed to increase the 

 opportunity for natural fireshwater flow events to enter the upper Nueces Delta. A key component of the 

 Rincon Bayou Demonstration Project was a 900-m long overflow channel which was excavated to coimect the 

 Nueces River with the extreme upper reach of Rincon Bayou, the dominant hydraulic feature of the upper 

 delta. A second overflow channel, which connected Rincon Bayou with a broad area of barren tidal flats in the 

 upper delta, was also excavated. These features effectively lowered the flooding threshold of the upper delta 

 and improved the distribution of diverted freshwater. 



If the historic flow regime characteristics of the upper Nueces Delta could be defined for the recent past, the 

 impacts of human activities, particularly that of reservoir development and management within the Nueces 

 River watershed, could be analyzed expUddy. These same components could also be used to evaluate the 

 potential alterations to the natural flow regime of the delta system resulting from a variety of restoration or 

 enhancement activities. 



Appendix C ♦ C-1 



