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11/14/01 draft report, Schmidt and Box flows on the middle Green River are increased and larval fish enter the Green River at much higher flows than are typical. We evaluated the sensitivity of larval fish populations to discharge of the Green River by predicting the longitudinal distribution of larval fish under a range of scenarios where a cohort of 100,000 fish enter the Green River on different days of 1992. The days at which fish were simulated to enter the river were between May 18 and July 17, and the discharge on those days varied between 45 and 207 m3s-1. We compared the predicted number of fish on September 15 throughout the study area and the proportion of that population located in reach 265250. The total population of larval fish in the study area is very sensitive to river discharge. The total number of fish in the study area on September 15 is predicted to vary between about12% of the total drift when drift enters the Green River at about 50 m3s-1 to about 1 % of the total driftwhen drift enters the Green River at about 200 m3s-1 (Fig. 14). The total number of fish predicted to occur in the study area is well predicted by y = 46882 - 20134 (log x) (5) where y is total number of fish predicted to occur in the study area on September 15 resulting from a cohort of 100,000 fish introduced into the Green River on 1 day and x is the discharge of the Green River, in cubic meters per second, on the day the fish first enter the river. The R2 of this relationship is 0.85. The proportion of larval fish occurring in reach 265250 increases with discharge from 0.16 to 0.22 for the same range of discharges. This relationship is y = 0.031782 + 0.083034 (log x) (6) where y is the proportion of the total number oflarval fish in the study area that occur in reach 265250 on September 15 and x is the same as in (5). The R2 of this relationship is 0.58. These simulations demonstrate that the timing of larvaLdrift in relation to discharge of the middle Green River is probably a critical determinant of the number of fish retained in the study area. The natural timing of drift is such that larval fish enter the middle Green River when river discharge is relatively low and shoreline complexity is high. In some years, however, dam operations result in artificially high river discharges in the middle Green River. These high discharges have the potential to transport larval fish beyond the study area. . The Value of Simulations It was necessary to make many assumptions in the process of developing the simulation model, and one might question the utility of this model. We were forced to make many assumptions about the characteristics of physical and ecological processes, because field data about these processes have not been previously collected. We were forced to make assumptions about the relative importance of physical and biological processes in controllingthe abundance and distribution of}arval pikeminnow. 16