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Last modified
7/14/2009 5:02:35 PM
Creation date
5/20/2009 10:36:59 AM
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UCREFRP
UCREFRP Catalog Number
9374
Author
Valdez, R. A. and B. R. Cowdell.
Title
Effects of Flow Regulation and Ice Processes on Overwinter Nursery Habitat of Age-0 Colorado Pikeminnow in the Green River Below Flaming Gorge Dam.
USFW Year
1999.
USFW - Doc Type
FG-10,
Copyright Material
NO
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<br />EXECUTIVE SUMMARY <br /> <br />Low survival of age-O Colorado pikeminnow (Ptychocheilus lucius) and low recruitment to <br />adulthood are primary factors that contribute to the endangerment of this indigenous Colorado River <br />Basin fish species. Low overwinter survival in the Green River, Utah, may be related to changes in <br />ice processes since completion of Flaming Gorge Dam in 1964. Ice conditions were evaluated in <br />nursery backwaters during winters of 1993-94 and 1994-95 to determine if availability and <br />persistence of these overwinter habitats are related to ice development and breakup processes linked <br />to dam operations. We tested the hypothesis that a post-dam decrease in frequency of stable river <br />ice cover has led to increased occurrence of ice jams and frazil ice that destabilize nursery <br />backwaters, causing the young fish to abandon these habitats at increased risk of predation and <br />presumed energy expenditure. <br /> <br />Ice conditions were studied in one of two primary nursery areas of Colorado pikeminnow for <br />two winters during special releases from Flaming Gorge Dam; i.e., high fluctuating releases in 1993- <br />94 and low stable releases in 1994-95. Both winters were relatively mild and ice development was <br />not as extensive as observed in previous colder winters. An ice cap was more persistent (42 days) <br />during low stable releases than during high fluctuating releases ( 5 days), providing evidence of a link <br />between dam operations and ice cap formation and persistence. These observations were consistent <br />"ith 4 7 years of historic records in which an ice cap formed on the middle Green River in 1 00% of <br />pre-dam years (18) when flow was relatively stable, but in only 48% of post-dam years (14 of 29) <br />under fluctuating hydropower releases. Warmer, high-volume, fluctuating dam releases have <br />precluded ice cap formation in the Green River nursery habitat area in all but the coldest winters, <br />disrupting the otherwise stable winter riverine environment created by a persistent ice cap and <br />naturally stable winter flows. <br /> <br />Despite the different ice cap conditions observed during the 2 years of special releases, the <br />percentage of backwaters that became unsuitable as nursery habitat in 1993-94 (50%) and 1994-95 <br />(56%) was similar. We attribute similar losses of backwaters to mild air temperatures in both <br />\vinters that precluded establishment of a thick, stable ice cap resulting in periodic and frequent ice <br />breakup. Of 14 and 9 backwaters surveyed in the two winters, 43% and 34%, respectively, became <br />flow-through channels as a result of ice jams, and 7% and 22%, respectively, were reduced in size <br />or depth by collapsing ice lenses or thickened shoreline ice. Backwaters were considered suitable <br />as long as they retained at least 30 m2 surface area, 0.3 m depth, were above -0.5 DC water <br />temperature, and had at least 5 mgIL dissolved oxygen-parameters believed to be important for <br />survival of age-O Colorado pikeminnow. <br /> <br />This study indicates that a stable ice cap is more likely to form on the Green River under <br />relatively stable low releases than under high-volume fluctuating flows during mild winters. <br />Additional studies are needed to determine if this relationship holds true during colder winters. <br />Maintenance of preceding summer and fall dam releases are recommended to maintain nursery <br />backwaters through the winter period. This flow recommendation is based on the need to minimize <br />the frequency of events in which ice is disrupted by large changes in release volumes and river stage, <br />\\hich can flood or desiccate nursery backwaters and displace the young fish. <br /> <br />iv <br />
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