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e CHAPTER n. RIVER GEOMORPHOLOGY The channel morphology is considerably different at the two study sites as is the flooded bottomlands habitat for endangered fish. At Ouray, overbank flooding creates shallow, low velocity, ponded waters with dense riparian and grasslands vegetation. The bottomlands flood waters are appreciably warmer than the river water and become rich with food organisms. Flooded bottom lands provide favorable conditions for phytoplankton production including nutrient availability, sunlight penetration of the water column and low velocity habitats (Stanford, 1993). Identified spawning beds for the razorback sucker are located approximately 50 miles upstream of the Ouray Wildlife Refuge. Historically, the flooded bottomlands provided vast areas of nursery habitat for the drifting larval fish. The first 50 miles of river channel below the spawning beds at RM 311, including the Ouray reach, has numerous floodplain areas which were frequently flooded. The extent of inundation was related to natural levee development, channel migration, bank erosion, sand bar development, sediment transport and river hydrology. e In Canyonlands the backwaters which provide nursery habitat for razorback sucker larvae are small, narrow channels associated with side canyons. When flooded by the Green River, the flooded habitat consists of backwater areas characterized by relatively short, narrow, deep, quiescent bodies of water. The extent of the backwater habitat depends on the river stage, but turbidity levels remain relatively high throughout the backwater. Water temperature gradients can be high from the river to the upstream extent of the backwater. The limiting feature of these backwater habitats in the Canyonlands reach is the narrow side channel mouth, which the larval fish can only access through the chance of floating near that river's edge when passing the side canyon. The canyon or valley geology which is the basis for the channel morphology and the bottomland or backwater habitat is discussed in the next section. Valley Geology - Ouray At the Ouray National Wildlife Refuge, the pinkish rocks that create the bluffs and cliffs along the river are the Uinta Formation from the Tertiary period. They include cross-bedded sandstone, conglomerate and unconsolidated siltstone and mudstone layers. The siltstone and mudstone layers are easily eroded. On the top of bluffs are cobbles and gravels transported by ancient streams from the Uinta Mountains (Goodknight and Ertel, 1987). The Green River has cut a series of meander loops through the Uinta Formation from where it leaves the Mesa Verde Formation south of Highway 40 to its confluence with the Duchesne. Near the highway, a series of river terraces can be seen which are evidence of former floodplains of large rivers that marked the advance and retreat of glaciers during the Pleistocene age (Chronic, 1990). The meander pattern that developed during this period has been retained as the river cut through the soft Uinta Formation. The most important characteristic of the Uinta Formation in the Uinta Basin is its e erodibility, which has contributed to the broad valley bottom and wide floodplains of the Green 12