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4.0 Prioritization Of Reaches And Sites 4-3 April 2004 An inventory of existing and potential floodplains during 1993-94 identified 132 sites with a potential of 18,430 acres on 393 miles ofthe mainstem Green River from Flaming Gorge Dam to the confluence of the Upper Colorado River (Irving and Burdick 1995). Of this total, 7,720 acres were inundated during the May high flow period (18,200 cfs) and 2,438 acres were inundated in the September low flow period (1,560 cfs). The maj ority of these sites were either shallow depressions (i.e., sites that hold water for a few weeks but fail to hold water year-around) or terraces (i.e., sites that flood and drain with river stage), and inundation may be from ground- water intrusion and not necessarily through surface connection with the river. Based on this inventory, the area of floodplains (and number of sites) in the Split Mountain to Desolation Canyon reach was 11,409 acres (37) at full inundation potential; 6,164 acres (35) at 18,200 cfs; and 1,660 acres (20) at 1,560 cfs (Table 4-1). Area of floodplain inundation in the Labyrinth and Stillwater canyons reach was 2,905 acres (44) at full inundation potential; 121 acres (22) at 18,200 cfs; and 5 acres (1) at 1,560 cfs. Area of floodplain inundation in the Gray Canyon to Labyrinth Canyon reach was 1,333 acres (21) at full inundation potential; 18 acres (5) at 18,200 cfs; and 0 acres at 1,560 cfs. Numbers of razorback sucker larvae potentially entrained in floodplains within each of the three priority reaches were estimated with the Floodplain Model (Valdez 2004) and used as indices of reach importance (Table 4-2, Figure 4-2). All model parameters were set equal and actual floodplain site locations were used for each reach (Irving and Burdick 1995). It was assumed that a spawning site was located 5 miles upstream from the upstream-most floodplain site of each reach, as is the case with the Split Mountain to Desolation Canyon reach. The location of each floodplain site relative to a spawning bar has a great influence on entrainment and accounts for reach differences in numbers of larvae entrained. These model simulations indicate that larval entrainment at full potential inundation was greatest in the Split Mountain to Desolation Canyon reach (1,322,380), followed by Gray Canyon to Labyrinth Canyon (685,276), and Labyrinth and Stillwater Canyons (379,460). Based on floodplain inundation at 18,200 cfs, estimated larval entrainment was far greatest in the Split Mountain to Desolation Canyon reach (1,322,371), and was similar to entrainment at full potential inundation because nearly all floodplains are inundated (although not necessarily connected) at 18,200 cfs. Entrainment estimates are not provided at 1,560 cfs because there is no surface connection between floodplains and the river at that stage. Reliable inundated floodplains and the presence of a known spawning bar in Split Mountain to Desolation Canyon highlight the importance ofthis reach in species recovery. The majority of floodplain area identified for each of the three river reaches is either shallow depressions or terraces that do not hold fish for long time periods for maximum growth and survival. Hence, estimates of larval entrainment do not necessarily reflect potential fish survival and recruitment from these floodplains.