Frequency 



The results of restored event return frequency Table 8: Summary of the restored return period for a flow event 

 varied depending upon the peak flow into the upper Nueces Delta with a daily peak flow of 100 acre-ft. 



considered (Figure 17). For example, for Restored Percent Change from 



Period Return Period Historic Period III 



(years) Conditions 



events with peak daily flows below 49 10' m' 

 (40 acre-ft), the estimated return period 



decreased to below historical levels of either III: 1982-1999 068 '324% 



Periodlorll. Above about 123 10' m' Note: 1 acre-ft = 12335 10' m' 



(100 acre-ft), the return period was greater than 



Periods I and II, but substantially less than that 



of Period III. If a peak flow event of 123 10' m' were used to illustrate the change in restored event return 



frequency from historical Period III conditions, the return period for this event would have decreased by about 



18 months, or by 324% (Table 8). 



Timing 



From inspection of Figure 18, and comparison with Figure 14, it is evident that the restored riming of freshwater 

 flow events into the upper Nueces Delta, both in seasonal and annual distribution, would improve from Period 

 III conditions. However, events xmder the restored condition would in no way attain the relative numbers or 

 size of flow events that occurred during histoncal times. Each of the visible events imder restored conditions, 

 although noticeably larger and longer, would still be considered only extremely small events in either historical 

 Periods I or II. 



DISCUSSION 



Historical Freshwater Inflow 



Without exception, a decreasing trend over time was observed in each of the four historical flow regime 

 characteristics analyzed. Event magnitude, duration, frequency and timing all declined with varying degrees from 

 Period I through Period III. When compared to Period I, which represents a conservative reservoir scenario, 

 flow regime characteristics during the period after the construction of Wesley Scale Dam (Period II) generally 

 exhibited a lesser degree of change than did the period immediately after the completion of Choke Canyon Dam 

 (Period III), which showed a more dramatic change. This would be expected, as Choke Canyon Dam provided 

 over twice the amount of storage capacity of Lake Corpus Chtisti and was operated in conjunction with the 

 latter, adding its effect upon the other. 



The magnitude of flow events during Period II decreased substantially from Period I levels, especially in regards 

 to the spring event. Dxmng Period III, event magnitude was virtually eliminated when compared to previous 

 periods. Changes in event duration closely reflected those in event magnitude, except that Period III levels did 

 not decreased as significantly. The differences between the return period of flow events into the upper delta 

 during Periods I and II were not as pronounced as with event magnitude and duration, but Period III again 

 showed a significant departure from previous periods. Finally, event timing dramatically changed with the virtual 

 elimination of meaningfiil seasonal flow events. This change was so distinct, that the entire post-Choke Canyon 

 Reservoir period (with the exception of 1987) could be likened to a perpetual "dry" year, which occurred only 

 infrequendy during the previous two periods. 



Potential for Restored Freshwater Inflow 



When compared with present conditions (represented by the historic Period III), each of the flow regime 

 characteristics analyzed under the restored Period III conditions were substantially increased. j\nnual event 

 magnitude was increased by over 633%, and annual event duration by over 578%. Event return frequency was 

 also improved, and even exceeded that which occurred in Periods I and II for lower peak flow events. Finally, 

 event timing also showed some improvement in the relative size and length of freshwater flow events. 



However, when compared with the historical flow regime of Periods I and II, the restored regime characteristics, 

 with the exception of event duration, compared less favorably. The restored event magnitude was still only a 

 fraction of even Period II levels, and event frequency was not affected for peak flow events greater than about 



Appendix C ♦ C-21 



