1^34 10' m' (1,000 acre-ft). Also, improvements in event timing were not sufficient to eliminate the significant 

 number of seasonal and annual "dry" periods in the Period III historical record. 



Comparison of Freshwater Flow into the Upper Nueces Delta Versus 

 Inflow into Nueces Bay 



This analysis was focused exclusively on flow event characteristics into the upper (northern) Nueces Delta, and 

 not on the total freshwater inflow into Nueces Bay. However, one minor digression worth consideration is the 

 difference between changes in the historical freshwater inflow pattern of the upper deha compared with that of 

 Nueces Bay. Clearly, flow into the upper Nueces Delta represents only a fraction of the river's total inflow to 

 the receiving bays and estuary. Consequendy, the results of a historical analysis of total estuarine inflow, without 

 consideration of the delta component, would be expected to differ considerably from the results of this 

 investigation. 



And this is indeed the case. Asquith el al. (1997) evaluated the mean and median annual stream flow of the 

 Nueces River near Mathis for the period 1940-1996, and performed statistical tests for historical trends with 

 time. Their results indicated that the change in stream flow from 1958 through 1982 (or Period II) was 

 negligible (an 0.8% decrease in mean annual flow, and an 18.5% decrease in median annual flow), while the 

 change in stream flow after impoundment of Choke Canyon Reservoir was large (a 55.0% decrease in mean 

 annual flow, and a 63.4% decrease in median annual flow). This conclusion is in contrast to the results of the 

 present analysis, which indicate the decrease in annual mean volume of water entering the upper Nueces Delta 

 during Period II was considerable (about a 39% reduction), and during Period III was extreme (over a 99% 

 reduction). 



This pragmatic explanation of this difference between the historic flow characteristics of the upper Nueces Delta 

 and those of the greater Nueces Bay Hes in the concept of "flooding threshold" at the point of natural diversion. 

 Because of the higher threshold imposed by the elevated river bank, freshwater diversions into the delta system 

 are extremely sensitive to the peak segments of flood events in the Nueces River. Therefore, if the flooding 

 threshold is not met, the flow event in the river will bypass the delta, providing Nueces Bay with an freshwater 

 inflow event without the same courtesy for the delta. Hence, changes in river flow patterns which lower the 

 peak flows of flood events disproportionately affect the upper delta as compared to the bay. 



The value of the distinction between delta inflow and total estuarine inflow is fully appreciated when recognizing 

 the fact that the Nueces Delta is a distinct and critical component of the greater estuary system. Without 

 consideration of this point, one may erroneously conclude that, for example, the reductions in freshwater inflow 

 during Period II did not meaningfully alter the freshwater flow regime of the Nueces Estuary ecosystem because 

 total mean inflow into Nueces Bay was reduced by only one percent. 



CONCLUSIONS 



The historic flow regime of the Nueces River into the upper Nueces Delta has changed dramatically over the 

 past 60 years. In each of the flow event characteristics analyzed, a strong declining trend was observed from 

 Period I through Period III. The Rincon Bayou Demonstration Project would restore a considerable amount of 

 freshwater to the upper Nueces Delta and significandy improve the flow regime characteristics compared to the 

 present (historical Period III) conditions. However, the demonstration project would not restore the delta's 

 inflow patterns to historic (Periods I and II) levels. 



Reservoirs in the basin and deltaic inundation events have a imique relation to peak flow events in the lower 

 Nueces River. On one hand, because of the high flooding threshold of the north bank of the river, the upper 

 Nueces Delta relies almost exclusively on the peaks of flow events for freshwater inflow. On the other hand, 

 main-stem reservoirs, by design, significandy attenuate the peaks of flow events in the watershed for purposes of 

 water storage and flood control. This relation is exemplified by the fact that, during the period after the 

 construction of Lake Corpus Christi (Period II), although annual mean flow of the river into Nueces Bay was 

 reduced by about 1% (Asquith et al. 1997), the annual mean flow into the upper Nueces Delta was reduced by 

 about 39%. Similarly, during the period after the construction of Choke Canyon Dam, while the annual mean 

 flow of the Nueces River into the bay was reduced by about 55% (Asquith et al. 1997), the annual mean flow into 

 the upper delta was all but eliminated (reduced by 99%). 



C-22 ^* Anatjists of the Historic Flow Eiffme of the Nueces River into the Upper Nueces Delta 



