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1 <br />POTENTIAL EFFECTS OF FOUR FLAMING GORGE DAM HYDROPOWER <br />OPERATIONAL SCENARIOS ON RIPARIAN VEGETATION <br />OF THE GREEN RIVER, UTAH AND COLORADO <br />by <br />K.E. LaGory and R.A. Van Lonkhuyzen <br />ABSTRACT <br />Four hydropower operational scenarios at Flaming Gorge Dam were <br />evaluated to determine their potential effects on riparian vegetation along <br />the Green River in Utah and Colorado. Data collected in June 1992 <br />indicated that elevation above the river had the largest influence on plant <br />distribution. A lower riparian zone occupied the area between the <br />approximate elevations of 800- and 4,200-cfs flows - the area within the <br />range of hydropower operational releases. The lower zone was dominated <br />by wetland plants such as cattail, common spikerush, coyote willow, juncus, <br />and carex. An upper riparian zone was above the elevation of historical <br />maximum power plant releases from the dam (4,200 cfs), and it generally <br />supported plants adapted to mesic, nonwetland conditions. Common species <br />in the upper zone included box elder, rabbitbrush, grasses, golden aster, and <br />scouring rush. Multispectral aerial videography of the Green River was <br />collected in May and June 1992 to determine the relationship between flow <br />and the areas of water and the riparian zone. From these relationships, it <br />was estimated that the upper zone would decrease in extent by about 5% <br />with year-round high fluctuation, seasonally adjusted high fluctuation, and <br />seasonally adjusted moderate fluctuation, but it would increase by about 8% <br />under seasonally adjusted steady flow. The lower zone would increase by <br />about 13% for both year-round and seasonally adjusted high fluctuation <br />scenarios but would decrease by about 40% and 74% for seasonally adjusted <br />moderate fluctuation and steady flows, respectively. These changes are <br />considered to be relatively minor and would leave pre-dam riparian vegeta- <br />tion unaffected. Occasional high releases above power plant capacity would <br />be needed for long-term maintenance of this relict vegetation. <br />1 INTRODUCTION <br />Rivers of the and and semiarid western United States generally have a distinctive <br />riparian zone where plant species typically found in moist soils are abundant (Knopf et al. <br />1988). Beyond this riparian zone, above the area affected by river flow, more xerophytic <br />species are present. These xerophytic species do not require the elevated soil moisture