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<br />-'aluatioDS
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<br />The evaluation performed to date of interim flow proposals has been limited
<br />to utilizing the best scientific and research data available and the best
<br />judgment of those involved in research efforts, recognizing that GeES are
<br />still in progress. In most instances, evaluations are necessarily limited
<br />to qualitative rather than a more desirable quantitative assessment. Further,
<br />it must be recognized that interim flows will be a short-term measure, pending
<br />completion of the EIS. Accordingly, assessment will be more limited than
<br />might otherwise be expected. Evaluations to date of the proposals have been
<br />made on the basis of the operating parameters in the paragraphs that follow.
<br />
<br />Maximum flow - The maximum release is based on two primary objectives: (1)
<br />to reduce erosion of elevated sand beaches, and (2) to limit the sand being
<br />transported out of the system. Another objective is to reduce impacts on
<br />Native American cultural sites. Maximum releases in the order of 20,000 cfs
<br />are supported by the information available as of July 21, 1991. The 20,000
<br />cts maximum is sufficient to make water deliveries of B.23 million acre-feet
<br />(mat) to the Lower Basin. Annual deliveries significantly over B.23 maf
<br />could require some upward adjustment in the maximum release. However, the
<br />probability of annual deliveries greater than B.23 maf is very low in 1992
<br />and into 1993. The limitation restricts peak energy production and precludes
<br />the use of available generating capacity except for emergency exceptions.
<br />
<br />Minimum flow - The minimum release is based on sufficient flow to: (1) reduce
<br />impacts on trout spawning and from stranding pools, (2) reduce impacts on
<br />native fishes, (3) reduce impacts on aquatic vegetation, (4) reduce impacts on
<br />vegetation and, (5) reduce impacts on recreation. A minimum flow of 5,000 cfs
<br />appears to meet these objectives. A minimum mean daily flow of B,OOO cfs has
<br />been recommended by the Ecological Resource Managers to support aquatic vege-
<br />tation and facilitate more effective habitat in backwater areas. Accomplish-
<br />ment ot these objectives could be achieved by adopting a B,OOO cfs minimum
<br />flow from 7 a.m. to 7 p.m. and no less than 5,000 cfs during the night. The
<br />changes would be made in accordance with adopted ramp rates.
<br />
<br />Dailv fluctuations - The reasons for limiting daily fluctuations are to:
<br />(1) reduce impacts on elevated beach erosion and associated sediment trans-
<br />port, (2) reduce impacts on fishes associated with spawning, stranding and
<br />backwater areas. From preliminary research data set forth in a supporting
<br />document of July 21, 1991, a daily fluctuation of 5,000 cfs would alleviate
<br />much, but not all, of the erosion impacts on elevated beaches. Some flex-
<br />ibility in daily fluctuation appears to be possible in recognition of stage-
<br />discharge relationships in accordance with the additional information
<br />documented as of July 21, 1991. A stage change limitation of 3 feet could
<br />restrict daily change to 5,000 cfs for maximum release in the magnitude of
<br />10,000 cfs. However, for flows of the 20,000 cfs magnitude, fluctuation up to
<br />B,OOO cfs fall within the 3 foot stage limit. Due to variability of cross
<br />sections, beach formations, and the location of beaches, there is a variance
<br />in stage-discharge relationships. Generally, a maximum fluctuation of 5,000
<br />cfs during low release months, 6,000 for medium release months, and B,OOO cfs
<br />for high release months would likely reduce beach erosion to tolerable levels.
<br />The daily fluctuation limitation restricts power demands, limits energy
<br />resource potential, and impacts energy commitments. The daily fluctuation
<br />limitation has the most severe impact on energy production of any of the
<br />interim flow parameters.
<br />
<br />RamD rates - Ramp rates, or the flow fluctuation over a l-hour period impact
<br />on: (1) erosion of elevated sand beaches and in other critical areas and, (2)
<br />trout spawning and stranding native fishes in the backwater areas. Ascending
<br />ramp rates of 4,000 cfs is an approximate threshold level above which impacts
<br />are accelerated. Reclamation proposed a limit of B,OOO cfs over a 4-hour
<br />period. This longer term limitation may, or may not, be important depending
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