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<br />Hourly and daily releases were modified in June 1990 when a schedule of research flows was <br />initiated. These flows were for the purposes of allowing the GCES researchers to evaluate the <br />impacts of current and alternative dam operations on the downstream resources for the EIS. <br />Following the research flows, the Secretary initiated interim operating criteria in August 1991 <br />IdeSigned to ameliorate the ra.te of adverse change on downstream resources resulting from past <br />dam operations pending a final decision on permanent long-term operating criteria. These <br />~ criteria, known as Interim Flows, are still in operation today. <br /> <br />J The annual volume of water released is influenced by regional hydrology and legal requirements. <br />\' In general, the types of water years and annual discharge in million acre-feet (mal) are classified <br />,,'f,. as: Lo~ - e.g. 8.23 maf, 1989 (the minimum discharge); Moderate - e.g. 13.6 maf, 1987; <br />d'~ and High - e.g. 19.3 maf, 1985. <br /> <br /> <br />J~ <br />'" . <br /><\:i O(J <br />It 'v~ <br />~~ <br />Q <br /> <br />Low water years are considered normal and likely to occur more than 50% of the time. Full <br />pool for Lake Powell is defined as 1,128 m and 24.3 maf of storage. Flood flows (flows greater <br />than 45,000 cfs) are estimated to occur for the No Action Alternative at a rate of 1 in 40 years. <br /> <br />Monthly volumes of releases vary depending on the type of water year and the Annual Operating <br />Plan. Under other than high water conditions, the greatest monthly releases have historically <br />occurred during peak energy requirement periods (summer and. winter). Daily and hourly <br />discharge rates are primarily a function of generation requirements. Daily fluctuations are <br />: generally highest during low volume years (power generated during high use periods of day and <br />reduced at night to save limited water supply) and decrease as yearly release volumes increase <br />(essentially extending peak period throughout day and into night). <br /> <br />The MLFF is similar to the original interim operating criteria with the exceptions that it does <br />not include the financial exception criteria allowed under Interim Flows but would include <br />habitat maintenance flows and increased flood protection (Table 1.). EIS elements common to <br />all alternatives also would be included. <br /> <br />13 Oc:lober 1993 Draa biological opinion 2,21-93-F-167 <br /> <br />5 <br />