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e e e The Ouray bridge just upstream of the Duchesne River in Photo 5 also constricts the river. Based on review of aerial photographs, sand bars located upstream of the bridge in 1963 have developed into a vegetated island. This island is several times larger than the sand bar visible in the 1963 aerial photos. At high flows the effects of the backwater from the bridge extend several miles up river. In the Canyonlands study reach, the narrow canyon confines the Green River floodplain and restricts the river's ability to migrate back and forth across the canyon bottom. The canyon pattern is a series of entrenched meander bends that were superimposed on the underlying formations. These incised meanders were inherited from an ancestral river that flowed across a mild-sloped floodplain. The general location of the meanders and sinuosity of the reaches has not been substantially altered during the incision process (Harden, 1990). The development of the floodplain has been relatively static in recent history. The river gradient in the Canyonlands study reach is 1.2 ft per mile or approximately 0.00023. The two study reaches, although dramatically different in river environment and canyon morphology, have virtually identical gradients. The geometry of the incised meander bends correlates with the resistance of the canyon geology (Harden, 1990). The erosion resistant White Rim formation appears to exert a strong influence on the canyon cross section shape (Harden, 1990). This also means that the river modified its channel geometry as it incised through the different formations. The cutoff meander bends occur more often at or near formation boundaries (Harden, 1990). This is the case for Anderson Bottom when the river drops from the more resistant White Rim Formation to the more erosive Organ Shale Formation. In addition, Harden (1990) argues that ingrown bends such as those in the study reach are the result of a mild slope which enhances lateral erosion. It can be construed from this discussion that a complex interaction occurs between cross section geometry, channel pattern and bedrock controls. Channel Cross-Sectional Geometry Active, migrating river channels carrying large sediment loads, have perimeter sediments that reflect the sediment being transported (Richards, 1982). Although primarily a sand bed, the Green River in both study reaches is not a typical alluvial stream with unlimited freedom to adjust its channel geometry. The perimeter of both channels has areas where bedrock occurs rather than sediments. At these locations, the bedrock contact influences the channel shape and geometry. The influence of bedrock is more predominant in the Canyonlands reach in which the entire meander pattern is incised in bedrock as opposed to the Ouray reach in which bedrock controls the width of the meander belt, but does not confine individual meanders. The river's numerous contacts with the bedrock geology in both reaches results in slope control, constrictions, ingrown bends into the bedrock, and constraints on channel incision. In both study reaches there are cross sections that are wide and shallow and cross sections that are narrow and deep. Where the river contacts bedrock on the outside of a meander bend, the river has attacked 16