WSP09202 - Laserfiche WebLink

Home Browse Search

c) lo"",:j i',J The model was completed with Input from the Bureau of Reclamation. Bureau of Indian Affairs, and the states of New MexIco and Colorado. , .\ 1-- 01 Mimicry of the natural hydrograph is the foundation of the flow recommendation process for the San Juan River. The !low recommendations require mimicry of statistical parameters of flow based on flow/geomorphology/habitat linkages and the statistical variability of the pre-dam hydrology rather than mimicry of each annual hydrograph. Therefore. the resulting flows will not mimic a natural hydrograph in all years, but will mimic the variation and dynamic nature of the 6S-year record of the San Juan River. The hyctrograph recommendations are designed to meet the conditions required to develop and maintain habitat for Colorado pikeminnow and razorback sucker and provide the necessary hydrologic conditions for the various life stages of the endangered and other native fishes, The conditions are listed in terms of flow magnitude. duration, and frequency during the spring runoff period. Duration is determined as the number of days that the specified flow magnitude is equaled or exceeded durin,g the spring runoff period of March I to July 31. Frequency is the average recurrence of the conditions specified (magnitude and duration), expressed as a percent of the 65 years of record analyzed (1929 to 1993), The underlying assumption in the fl()w conditions is that, over a long period of time, history will repeat itself: if the conditions were met during the past 65 years, they will also be met in the future. To the extent that the water supply is different in the future, then the natural condition would also be altered and the conditions of rrii mi cry would be maintained, although the exact flow recommendation statistics may not be met. To allow for gage and modeling error and the difference between the flows at the historical gage at Bluff. Utah, and the Four Corners gage. maximum allowable durations are computed for 97% of the target flow rate. In most cases, the primary recommendation is for a specified flow rate (i,e", 10,000 cubic feet per second (efs)) of a minimum duration (i,e., S days) for a specific frequency of occurrence (i,e" 20% of the years), In addition to the primary recommendation, variability in duration is desirable to mimic a natural hydrograph, Therefore, a frequency table for a range of durations for each flow rate is recommended. A maximum duration between occurrences is also specified to avoid long periods when conditions are not met, since such long periods could be detrimental to the recovery of the species, The maximum period without reaching a specified condition was determined as twice the average required interval (except for the 80% recurrence of the 2.500 cfs condition. where 2 years is used), For example, if the average interval is I year in 3, then the maximulll period between meeting conditions would be 6 years, The maximum periods were based on the collective judgement of Biology Committee members after review of historical pre-darn statistics. Following are the conditions specified: A, Category: Flows> 10.000 cfs during runoff period (March I to Julv 31). Duration: A minimum of 5 days between March 1 and July 31. SJRIP Biology Committee February 1999 S-5 Executive Summary Flow Report