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6 3 RESULTS 3.1 VEGETATION OF RIPARIAN ZONES The width and height of the riparian zone as measured in different reaches of the Green River is presented in Table 1. No clear difference between canyon and park areas was detected from these data. The riparian zone was narrowest in Red Canyon just below Flaming Gorge Dam, but on average was widest in Lodore Canyon. The upper boundary of the riparian zone was lowest in Browns Park, but relatively high in Island and Rainbow Parks. Fifty-four plant species were identified at 2,165 sampling points along the transects; an additional 14 taxa were identified to genus or family (plant nomenclature follows Goodrich and Neese 1986). The relative abundance of species and taxa is presented in Table 2. Grasses, which were recorded at 44% of all sampling points, were the most common of all taxa recorded but were not identified to species. Other common species in the river corridor included scouring rush (19%), juncus (14%), western goldenrod (12%), common spikerush (11%), box elder (11%), carex (9%), tamarisk (7%), field horsetail (M), and coyote willow (7%). (See the appendix, Table A.4, for the scientific names and wetland indicator status [Reed 19881 of all species observed.) For most species, abundance varied considerably among reaches, especially between canyon and park reaches (Table 2). Species most common in canyons included box elder (especially Lodore Canyon), western goldenrod, and scouring rush. Species more common in the parks than in the canyons included common spikerush, coyote willow, and Fremont cottonwood. Both distance from and elevation above the river were important determinants of species abundance (Tables 3 and 4). Presumably these factors were important insofar as they determined soil moisture, and species-specific requirements for moisture and flood tolerance determined where those species occurred on the distance/elevation gradient. This interpre- tation is supported by the observed position of each species along the elevation gradient and their wetland indicator status (see Table AA) - obligate wetland species (i.e., species that occur almost always in wetlands) were found closer to the river, whereas facultative wetland species (i.e., wetland species sometimes found in uplands) or nonwetland species were found farther from the river. As might be expected, elevation appeared to have a stronger influence than distance on species occurrence because of the effect of elevation on depth to the water table. Some woody riparian species (e.g., box elder and rabbitbrush) increased in abundance with increasing distance and elevation; others (e.g., coyote willow and tamarisk) appeared to occupy a specific elevation midway between the lower and upper boundaries of the riparian zone. Wetland species decreased in abundance with distance and elevation (e.g., cattail, common spikerush, and coyote willow) or were more prevalent at a particular elevation or