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<br />involve a long-tenn change in operations to provide for a spring peak and maintenance of a base <br />flow. The base flow will cause releases to be generally lower than presently occur, although at <br />certain times during the summer, releases will be increased over existing conditions to meet base <br />flows. <br /> <br />Flow recommendations were developed using U.S. Geological Survey (USGS) flow data records <br />from 1929 to the present. Prior to Navajo Dam, approximately 73 percent of the total annual <br />flow occurred during spring runoff (March-July) (Bliesner and Lamarra, 2000) with mean daily <br />peaks at Bluff UT ranging from 3,810 to 33,800 cfs and averaging 10,500 cfs. The highest peaks <br />have actually occurred during summer and fall thunderstonns and reached an estimated 70,000 <br />cfs at BluffUT in September, 1927 (USGS data). Prior to Navajo Dam annual flow was quite <br />variable, ranging from 618,000 acre-feet (at) to over 4,000,000 af at Bluff; median annual flow <br />was 1,620,000 af. Summer low flows occasionally reached zero pre-dam and monthly mean <br />flows were as low as 65 cfs. While low summer flows are a natural condition, these very low <br />flows occurred due to a combination of irrigation diversions and dry periods. Following <br />operation of Navajo Dam, spring peaks were substantially decreased (post-dam peaks averaged <br />54 percent of pre-dam peaks) and flows for the remainder of the year increased (August-February <br />flows averaged 168 percent higher ). <br /> <br />Beginning in 1991 test flows for endangered fish were provided from Navajo Dam, including <br />spring releases of 5,000 cfs to increase downstream peaks. This test period showed that <br />operation changes could successfully better mimic a natural hydrograph (see attachment D for <br />graph of flows during test period). The test period also showed that the cooling effect of Navajo <br />Dam on water flows extended further downstream with the high releases. <br /> <br />Tables and figures on the following pages compare reservoir elevations and river flows between <br />a No Action alternative and the proposed plan. Figure 2 tracks average water surface elevation at <br />Navajo Reservoir under both alternatives. Table I estimates flow levels at different gage <br />locations on the San Juan and Animas Rivers under the alternatives. Figure 3 gives an indication <br />on the frequency certain flows would be expected at the Archuleta gage under the alternatives. <br /> <br />The Archuleta gage is approximately 6 miles downstream from the dam and approximately I <br />mile upstream from the Citizens' Ditch, a major diversion. Thus, flows would be much lower <br />downstream from the Citizens' Ditch than shown at the Archuleta gage. Lowest flows would <br />occur just downstream from the Hammond Diversion Dam, approximately 20 miles downstream <br />from the dam. During the 2001 Low Flow Test (Dam releases were lowered to 250 cfs), flows <br />fell to 133 cfs downstream of the Citizens' Ditch and 63 cfs downstream from the Hammond <br />Diversion. Under actual low flow conditions, these numbers may be lowered. <br /> <br />Other gages are located in the critical habitat reaches of the river from Fannington to Lake <br />Powell. <br /> <br />10 <br /> <br />00684 <br />