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1 t 1 1 t Canyonlands Geology Canyonlands Geology and River Geomorphology The narrow canyon corridor in Canyonlands National Park confines the Green River to a narrow floodplain and limits its migration across the canyon bottom. The canyon is a series of entrenched meander bends that were superimposed on the underlying geologic formations (Photo 1). These incised meanders were inherited from an ancestral river that flowed across amild- 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 geology of the Canyonlands reach of the Green River consists sedimentary rocks exposed by numerous periods of plateau uplift and river down cutting. The stratigraphic record is a series of alternating hard and soft formations forming mesas, buttes, cliffs and steep canyon walls. The most prominent geological formation in Canyonlands is the thick reddish brown sandstone cliffs of the Wingate Sandstone. At river mile 37, the top of the White Rim Sandstone emerges at the river level and becomes the dominant formation in the study reach (Photo 1). Underlying the White Rim Sandstone is the Organ Rock Shale, a soft reddish brown shale, which begins at about river mile 32 near the location of the gage. Amore complete discussion of the canyon geology is presented in the FLO Engineering report (1995b). River Geomorphology Although primary a sand bed, the Green River in Canyonlands can not be considered an alluvial stream with the freedom to adjust its channel geometry. The river's numerous contacts with the bedrock geology in this reach results in slope control, constrictions, ingrown bends into the bedrock, and constraints on channel incision. The average river gradient is 1.22 ft per mile or approximately 0.00023. The historic river channel had several wide reaches with active sand bars which have become encroached with vegetation and attached to the river banks. In addition, the floodplain terrace near the river is now densely vegetated with old tamarisk trees, willows and other riparian vegetation that has cutoff access from the river to the upland vegetation zones. Some of the floodplain bottoms such as Queen Anne, Anderson, and Valentine have a few old cottonwoods or some small cottonwood stands. Most of the older sand bars, islands and terraces have exceedingly dense vegetation. The floodplain terraces are hydrologically disconnected from the river and the development of the floodplain has been static since about 1930 (FLO, 1995b).