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360 flow occurred during spring runoff (March-July) (Bliesner and Lamarra, 2000) with mean daily 361 peaks at Bluff UT ranging from 3,810 to 33,800 cfs and averaging 10,500 cfs, The highest peaks 362 have actually occurred during summer and fall thunderstorms and reached an estimated 70,000 363 cfs at Bluff UT in September, 1927 (USGS data). Prior to Navajo Dam, annual flow was quite 364 variable, ranging from 618,000 afto over 4,000,000 afat Bluff; median annual flow was 365 1,620,000 af. . Summer low flows occasionally reached zero pre-dam and monthly mean flows 366 were as low as 65 cfs. While low summer flows are a natuqil condition, these very low flows 367 occurred due to a combination of irrigation diversions and dry. periods. Following operation of 368 Navajo Dam, spring peaks were substantially decreased (post-dam peaks averaged 369 54 percent of pre-dam peaks) and flows for the remainder of the year increased (August-February 370 flows averaged ]68 percent higher). 371 Beginning in 1991 test flows to determine effects on endangered fish were provided from Navajo 372 Dam, including spring releases of 5,000 cfs to increase downstream peaks. This test period 373 showed that operation changes could successfully better mimic a natural hydrograph (see 374 Attachment D for graph of flows during test period). The test period also showed that the 375 cooling effect of Navajo Dam on water flows extended further downstream with the high 376 releases. 377 Tables and figures on the following pages compare average reservoir elevations and river flows 378 between the No Action alternative and the proposed plan. Figure 2 tracks average water surface 379 elevation at Navajo Reservoir under both alternatives. Table I estimates flow levels at different 380 gage locations on the San Juan and Animas Rivers under the alternatives. Figure 3 gives an 381 indication on the frequency certain flows would be expected at the Archuleta gage under the 382 alternatives. Under the proposed plan, average monthly releases of 250 cfs would occur 28 383 percent of the months in the winter and 29 percent of the months in the March thru November 384 period. Overall monthly releases would be below 500 cfs 98 percent of the time in the winter 385 and 65 percent of the time the remainder of the year. 386 The Archuleta gage is approximately 6 miles downstream from the dam and approximately I 387 mile upstream from the Citizens' Ditch, a major diversion. Thus, flows would be much lower 388 downstream from the Citizens' Ditch than shown at the Archuleta gage. Even lower flows would 389 occur just downstream from the Hammond Diversion Dam, approximately 20 miles downstream 390 from the dam. During the 2001 Low Flow Test (Dam releases were lowered to around 250 cfs), 391 flows fell to,133 cfs downstream from the Citizens' Ditch and 63 cfs downstream from the 392 Hammond Diversion. Under actual low flow conditions, flows may be less or greater than these 393 amounts. Table 2 summarizes river flows measured during the Low Flow Test 394 395 The No Action alternative does not mimic the natural hydrograph and it is assumed that projects 396 that rely on such mimicry for ESA compliance (for example, Animas-LaPlata and completion of 397 NIlP) would not occur. Under No Action, it is assumed that Navajo Dam would be operated as it 398 was from 1973 to 1991, with a minimum release of 500 cfs. Operational goals include to store as 399 much water in the reservoir as possible and to maintain uniform flows downstream. 13