8255

• UCREFRP Catalog Number: 8255
• Author: Grams, P. E. and J. C. Schmidt.
• Title: Geomorphology of the Green River in the Eastern Uinta Mountains, Colorado and Utah.
• USFW Year: n.d.
• USFW - Doc Type: Logan, Utah.
• Copyright Material: NO

ABSlRACf
Longitudinal prof1le, 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 correlated 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 correlated
with river-level lithology that is of moderate to low resistance and very low debris fan
frequency. The channelis 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 3 borehole sites. Thus the influence of lithology on
channel form is indirect. The influence of lithology primarily operates through the
presence of resistant boulders in debris fans that are delivered by debris flow from steep
tributaries Shear stress estimates indicate that bed material size and channel form and
steepness are in approximate adjustment for discharges of about the 10-yr recurrence
flood as determined for unregulated streamflow. Downstream transport of gravel is
limited; gravel-bar lithology shows a strong relationship to the source lithology of the
adjacent upstream debris fan. These observations suggest that the Green River through
the eastern Uinta Mountains has been dominated by aggradation during recent geologic
time.

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.