9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

1988; Pucherelli et al., 1990). Based on the response of the Green River to dam emplacement and the
reported responses of other rivers to similar sedimentologic and hydrologic changes (Williams and Wolman,
1984), it is reasonable to conclude that similar morphologic responses could result from installation of dams
in the alluvial reaches of the Yampa River system.

Canyon reaches of the Colorado Plateau rivers can be considered to be bedrock bounded and as
such they are fundamentally different from alluvial reaches (Ashley et ai., 1988). Bedrock-channel
morphology is regarded as a function of the physical characteristics of the bedrock rather than hydraulic and
sediment transport characteristics of the river. Harden (1990) investigated the controlling factors on
bedrock-bounded (canyon) reaches of the Green, San Juan and Colorado Rivers and concluded that average
gradient, bedrock resistance to erosion and regional structure all affect the geometry and spatial distribution
of meanders. Changes in sediment supply and hydrology consequent on dam emplacement are unlikely to
cause significant changes to meander geometry in any but the very longest time scale. Reduced sediment
supply to the canyon reaches could in the long run lead to accelerated rates of bedrock erosion, but these
changes are beyond any consideration within an engineering time scale. However, even though the macro-
scale geometry of the canyon reaches is unlikely to be affected by changes associated with dam
emplacement, the geometry is still important on a meso-scale because of its controls on the spatial
distribution of meso-scale features such as rapids, riffles, pools and bars (Lisle, 1986; O'Connor et al., 1986).

6.1.2. Meso-Scale MOI:pholOiV and Response. The spacing and geomorphic significance of rapids
within the canyon reaches of the rivers draining the Colorado Plateau has received considerable attention
because rapids, which make up only about 10 percent of the rivers' lengths, account for about half of the
elevation drop (Leopold, 1969). Rapids pose significant navigation hazards and where the rapids are
associated with tributaries supercritical flow conditions may occur at certain discharges (Kieffer, 1985).
Elimination of very large magnitude flows as a result of dam emplacement can result in increased
contraction of main stem sections where the contractions are due to tributary fans. Kieffer (1985) concluded
that the average contraction ratio at tributary fans on the Colorado River decreased from 0.5 prior to dam
construction to 0.3 after dam construction because the controlled flows were no longer able to erode the
debris fans. The reduced fan erosion had 2 consequences: 1) there were less bars downstream of the fans
and 2) the navigation hazards increased as well.

Rapids on the Colorado River have been reported to be spaced regularly (Leopold, 1969), or with
both regular and random components (Graf, 1979). The spacings of the rapids has been attributed to
intrinsic adjustment of sediment distribution toward maintaining a quasi-equilibrium longitudinal profJIe
(Leopold, 1969), to tributary debris fans that constrict the river (Howard and Dolan, 1981), or to a
combination of tributary debris fans or other sources and reworking of the deposits by high discharges on
the main stem (Graf, 1979). Webb et al., (1988) concluded that 95 percent of the major rapids in the Grand
Canyon National Park section of the Colorado River were related to episodic debris flows from tributary
drainages, about 4 percent were related to the development of bars below tributary mouths and the
remaining 1 percent were related to rockfalls from the canyon walls. Webb et al.'s study showed that small
drainages can exert significant control on the meso-scale morphology of much larger rivers. Reduction of
peak flows as a result of dam emplacement is likely to magnify the local controls on channel gradient that
are exerted by the smaller tributary drainages and as such they could have greater effects on sediment
transport and storage in the higher order rivers.

In alluvial channel reaches alternating deeps (POOls) and shallows ( riffles) are characteristic of both
straight and meandering channels with heterogeneous bed materials in the range of 2 to 256 mm (Knighton,
1984). Pool-riffle spacing is in the range of 5 to 7 times the channel width and the sediments that comprise
the riffles are coarser than those that are located in the pools. Under low flow conditions the flow velocity is
highest over the riffles and lowest over the pools. At higher discharges there is a velocity reversal and the
highest velocities are located within the pools. This phenomenon is significant with respect to the
distribution of sediment sizes with stage. Under low flow conditions the fmer sediments are concentrated in
the pools, but under higher discharge conditions the coarsest sediments are transported though the pools
and these sediments are then deposited on the riffles where the velocities are lower (Lisle, 1979). Pool-riffle

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