Laserfiche WebLink
Canyon compared with > 20°C in the pre-impoundment period (Fig. 3 in Bestgen and Crist <br />2000). Post-dam winter water temperatures in the tailwater were increased compared to pre- <br />impoundment conditions from near 0°C to about 4°C. <br />Cold tailwater releases begun after 1967 dramatically reduced trout growth in the fishery <br />that established after dam closure, which prompted the second major Green River change due to <br />Flaming Gorge Dam (Holden and Crist 1981). A multi-level penstock was installed and became <br />operational in June 1978 (mean daily water temperature was 5°C on 19 June, increased to 13°C <br />on 22 June) to allow water to be drawn from wanner upper layers of the reservoir in summer. <br />Temperature of multi-level penstock releases during summer operations were targeted at about <br />13°C, a level thought to maximize growth of rainbow trout Oncorhynchus mykiss and maximize <br />downstream extent of the tailwater fishery. Effects of penstock modifications on native fishes in <br />reaches further downstream were assessed from 1978 to 1980 by Holden and Crist (1981). <br />Penstock modifications raised water temperature in the Green River upstream of the Yampa <br />River, but summer maxima rarely exceeded 17°C in the period 1978 to 1991 (Fig. 3 in Bestgen <br />and Crist 2000). <br />The third major dam-related operation event occurred in 1992 in response to a Biological <br />Opinion on operation of Flaming Gorge Dam (Tyus and Karp 1991, U. S. Fish and Wildlife <br />Service 1992). Spring release levels from the dam were increased in an attempt to simulate a <br />more natural runoff pattern and enhance floodplain inundation downstream of Jensen, Utah (Fig. <br />2). The spring through autumn flows implemented in 1992 were designed to enhance physical <br />habitat in the Green River downstream of the Yampa River confluence; habitat enhancement in <br />the regulated Green River reach upstream of the Yampa River was not a priority because few <br />endangered fishes were thought to occur there. Releases up to maximum power plant levels (130 <br />m'/sec, Fig. 4) were made for up to six weeks in spring around the time when unregulated <br />Yampa River discharge peaked. During summer, release levels were designed to maximize <br />nursery habitat for Colorado pikeminnow near and downstream of Jensen, Utah (Tyus and Karp <br />1991). To achieve the desired stable base flow of 51 m3/sec (+ 12.5%, Pucherelli et al. 1990, <br />15 <br />