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Final Report 3-48 September 2000 Reach 3, about 1,100 ha was present in the portion of the reach between the White River confluence and Pariette Draw and about 760 ha was present in Canyonlands. Irving and Burdick (1995) prioritized these bottomlands according to their value to endangered fishes and concluded that the highest priority bottomlands were in Reach 2 and upper Reach 3 (Escalante Ranch to Pariette Draw). They did not determine the relationship of floodplain inundation to flow. Several areas along the Green River, including portions of the Ouray National Wildlife Refuge (NWR), Dinosaur National Monument, Canyonlands National Park, and a significant portion of Reach 2 in the Uinta Basin, have been studied to determine the relationship of flow to floodplain inundation (FLO Engineering, Inc. 1996, 1997; Bell et al. 1998; Cluer and Hammack 1999). Figure 3.18 illustrates this relationship in different study areas. The greatest area of floodplain habitat suitable for satisfying the life-history requirements of endangered fishes in the Green River system is located in the Ouray portion of Reach 2. The investigation of floodplain-habitat inundation in the Ouray portion of Reach 2 by FLO Engineering, Inc. (1996) used the U.S. Army Corps of Engineers HEC-2 step backwater model to estimate water-surface profiles for a segment of river from RK 400 to 427. The model was used to estimate bankfull discharge and the relationship between flow and area of inundation. Under existing conditions at Ouray, the amount of floodplain inundation begins to increase rapidly as flows exceed about 527 m3/s. With existing artificial levees removed, flooding would be initiated at flows between 368 and 453 m3/s. Flooding in the Old Charlie Wash area, where a small side channel allows flooding of approximately 250 ha, begins at approximately 368 m3/s. Other floodplain areas have inlet structures that are operational at flows of 85 to 113 m3/s. If 0.6- to 0.9-m-deep side channels were excavated at certain locations, flooding of about 2,185 ha could be initiated in Ouray floodplain habitats at 368 m3/s. Flooding in the Island Park portion of Reach 2 (RK 530 to 535) was investigated by Cluer and Hammack (1999). They used numeric hydraulic modeling to evaluate the availability of various habitats over a range of flow conditions at this location. Inundated floodplain area increased from about 2 ha at 312 m3/s to about 98 ha at 1,090 m3/s. Bell et al. (1998) used aerial photography to determine the relationship between flow and floodplain inundation in Reach 2 from Split Mountain Canyon to the White River (RK 396 to 505) and the upper portion of Reach 3 from the White River to Pariette Draw (RK 380 to 396). At 566 m3/s, about 2,100 ha were flooded in Reach 2, about 3,500 ha were flooded at 624 m3/s, and about 4,900 ha were flooded at 705 m3/s. Most of the floodplain habitat in Reach 3 is located in the upper portion of the reach just downstream of the confluences with the White and Duchesne Rivers, and this habitat is contiguous with the extensive floodplain habitats of Reach 2. In the upper portion of Reach 3 examined by Bell et al. (1998), the area of floodplain inundation was 265,425, and 767 ha at 623, 680, and 920 m3/s,respectively, asmeasuredatthe USGS gage near Green River, Utah (Figure 3.18). Downstream of this area, the river channel is more confined because of either resistant geology (Desolation, Gray,