Geomorphology of the Green River in Dinosaur National Monument
Longitudinal profile, channel cross-section geometry, and depositional patterns of the Green River in its course through the eastern Uinta Mountains are each strongly influenced by river -level geology and tributary sediment delivery processes. We surveyed channel cross sections at 1-km intervals, mapped surficial geology, and measured size and characteristics of bed material in order to evaluate the geomorphic organization of the 70- km study reach. Canyon reaches that are of high gradient and narrow channel geometry are associated with the most resistant lithologies exposed at river level and the most frequent occurrences of tributary debris fans. Meandering reaches that are characterized by low gradient and wide channel geometry are associated. with river-level lithology that is of moderate to low resistance and very low debris fan frequency. The channel is in contact with bedrock or talus along only 42 percent of the bank length in canyon reaches and there is an alluvial fill of at least 12 m that separates the channel bed from bedrock at three borehole sites. The influence of lithology primarily operates through the presence of resistant boulders in debris fans that are delivered by debris flows from steep tributaries.
The depositional settings created by debris fans consist of (1) channel-margin deposits in the backwater above the debris fan, (2) eddy bars in the zone of recirculating
flow below the constriction, and (3) expansion gravel bars in the expansion below the zone of recirculating flow. These fan-eddy complexes are the storage location of about 70 percent. by area, of all fine- and coarse-grained alluvium contained within the canyons above the low-water stage. Immediately adjacent meandering reaches contain an order of magnitude more alluvium by area but have no debris fan-created depositional settings.
This study also describes the flood-plain and terrace stratigraphy of the Green River in the eastern Uinta Mountains and changes due to the operations of Flaming Gorge Dam, upstream from the study area. These landforms are vertically aggrading deposits that are longitudinally correlative throughout the 65-km study reach. The suite of surfaces identified includes a terrace that is inundated by rare pre- or post-dam floods, an intermediate bench that is inundated by rare post-dam floods, and a post-dam flood plain that is inundated by the post-dam mean annual flood. Analysis of historical photographs in the study reach shows that both the intermediate bench and post-dam flood plain are landforms that were not present in any of the 6 years for which photographs were examined between 1871 and 1954. Photographic replications also show that gravel bars consisting of bare gravel in 1922 and earlier photographs are now covered by fine-grained alluvium and vegetation . Decreased gravel-bar mobility is indicated by estimates of critical and average boundary shear stress. Comprehensive surficial geologic mapping of the study area indicates that the bankfull channel has decreased in width by an average of about 20 percent.