9612

• UCREFRP Catalog Number: 9612
• Author: N.A.
• Title: Flaming Gorge Flow Recommendation Investigation FG-1 Technical Integration and Final Report - Hydrology and Physical Science Chapter - Draft.
• USFW Year: 1998.
• USFW - Doc Type: \
• Copyright Material: NO

• DRAFT FeMvary 25, 1998
Canyon (reach 3), and canyons with abundant debris fans are exemplified by the geomorphic
conditions of Lodore Canyon (reach 1) and Gray and Desolation Canyons (reach 3).
Characteristics of Restricted Meandering and Fixed Meandering Reaches
Restricted meandering and fixed meandering reaches of the Green River exhibit similar physical
characteristics. Typical elements of these reaches are the channel, vegetated islands, unvegetated
bank-attached compound bars, unvegetated island-attached compound bars, and unvegetated
mid-channel compound bars. Permanent islands are rare in fixed meandering reaches. Thalweg
and bar substrate is typically sand, although gravel bars occur in some segments. Typically,
bank-attached compound bars occur on alternating sides of the river. Shoreward from these bars
is the vegetated floodplain at the edge of the bankfull channel, and streamward from the bars is
the meandering thalweg. Island-attached compound bars are bounded by vegetated islands and
the thalweg.
At low discharge, exposed compound bars have an irregular topography caused by chute channels
that dissect the bar platform. Chute channels are oriented in a downstream direction, crossing
from streamward to shoreward side at the upstream end of the bar and from shoreward to
streamward side at the downstream end of the bars. Topography is more complex where there .
are more chute channels. At some sites and in some years, secondary bars become attached to the
shoreward margins of these compound bars. At the downstream end of most compound bars,
chute channels may converge into one persistent and deep secondary channel that separates the
downstream end of the compound bar from the floodplain. This secondary channel is typically so
deep that it constitutes a slackwater at very low discharges. The remainder of the bars are
composed of broad, level platforms and linear ridges that may be partly vegetated.
Stagnant and recirculating flow develops at low discharge in the lee of exposed parts of these
compound bars. As flow recedes from the annual peak discharge, parts of the bar platform with
higher elevations are exposed, and small areas of separated flow develop in the lee of these
islands; at these discharges, chute channels actively transport sediment. Upon further recession of
flow, chute channels at the upstream end of the compound bar become exposed, and flow in the
secondary channel ceases; thereafter, the secondary channel becomes an area of stagnant water.
Continued emergence blocks flow in chute channels in the downstream part of the compound bar.
Allred (1997) described the process of vertical accretion of inset floodplain deposits. The
formative steps identified by the author include: 1) emplacement and accretion of a lateral bar as
large amounts of sediment are being moved through the system; 2) low flood magnitude in years
following bar emplacement; 3) rapid encroachment of riparian vegetation onto the bar surface; 4)
stabilization of the bar through extensive root system development; and 5) continued vertical
accretion of the bar surface during episodic periods of inundation.
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