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11/ 14/01 draft report, Schmidt and Box Hydraulic factors primarily involve the exchange rate of water between the main flow and backwaters, and field studies of the longitudinal dispersion of dye can be applied, because the behavior of floating or neutrally-buoyant particles is similar to that of dye clouds (see Fischer 1973, table 1). The exchange of water with backwaters, called "dead zones" in fluid mechanics literature, is responsible for the time lag between passage of the peak concentration, centroid of mass, and trailing edge of longitudinally-dispersing dye clouds in natural rivers (Fischer et al. 1979). Skewness of the dye cloud increases as the size and number of dead zones increases (Valentine and Wood 1977). Thus, increased number of backwaters should lead to slower average drift rates, which has been shown on the Pecos River in New Mexico where the rate of downstream drift is slower in reaches with complex shoreline topography (Dudley and Platania 2oo0b). Some parts of the middle Green River include large recirculating eddies that are similar to those that occur in the Colorado River in Grand Canyon (Schmidt and Rubin 1995). Graf (1995) showed that recirculating eddies in the Grand Canyon are not dead zones and that dye clouds display less longitudinal dispersion in Grand Canyon than in alluvial rivers. The exchange rate of water between eddies and the main flow increases as discharge increases, and skewness of the main dye cloud decreases during floods (Konieczki et al. 1997). Backwaters used by Colorado pikeminnow are within-channel features and do not occur on the floodplain, because drift occurs during the receding limb of the annual flood. Backwaters occur along crenulated banks at flows near bankfull, and backwaters occur in the embayments of irregularly-shaped alluvial sand bars at moderate and low flows (Fig. 3). Thus, the number, size, and characteristics of backwaters at the time of larval drift are primarily linkedto the topography and distribution of alluvial bars (Rakowski 1997). Backwaters are typically larger and more frequent in wide alluvial reaches between Jensen and Ouray, and sampling pi\~~~ish in late summer and fall is concentrated there (Muth et al.20oo). '\ \JJr';<"~ Hydrology and Drift between 1990 and 1995 \ rw \ l./ The 6 years of field data span a range of flow and drift conditions. Runoff was greatest in 1995 and was least in 1992 and 1994 (Table 1). Larvae began entering the Green River later in 1~5 than in other years, and the discharge at that time was about 300 m3s't, at least 3 times more than in other years (Fig. 4). Runoff was nearly as large in 1993, but larvae entered the Green River afterrecession to basenow. The number of drifting larvae in 1991, 1992,1993, and 1995 was between 500,000 and 700,000 (Table 2). In 1994, there was an order of magnitude fewer drifting. - . larvae. In 1990, there were 1,100,000 drifting larVae. Thus, 1994 was a year of very low discharge and few larvae, 1992 was a year of very low discharge and an average number of larvae, 1995 was a year of very large discharge and an average number of larvae, 1990 was an average runoff year with a very large number of larvae, and 1991 and 1993 were years when runoff and the number of larvae were about average. rJ., y.' \ 'l \" J 1 ;---- ' 6 IIIl...