7197

• UCREFRP Catalog Number: 7197
• Author: O'Brien, J. S.
• Title: 1983 Yampa River Cobble Reach Morphology Investigation Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

• • 13
The incised meanders have an average wavelength of approximately
0.62 miles. The original formative discharge for producing the
paleochannel meanders with a wavelength of 0.62 miles is approximately
10,500 cfs. This is derived from empirical relationships of existing
rivers (Richards, 1982). The corresponding width is about 250 feet or
equal to present day width. Over geologic time the annual discharge has
increased.. As the base level dropped and the river incised in bedrock,
the drainage basin developed increasing the annual discharge.
Every river system evolves in a manner that establishes approximate
equilibrium between the channel and the water and sediment it conveys.
The reaches in the canyon comprise the profile shown in Figure 2 which
reflects the long term evolution on geologic temporal and spacial
scales. In the upper reaches of the canyon, the river has incised in
the Morgan shale formation. Boulders and talus from steep side slopes
comprise most of the bed material. Downcutting is inhibited by the
large substrate and the erosion resistant formation.
The slope-substrate-discharge relationship is complicated. Size of
the bed material is proportional to the depth and slope and generally, a
milder slope will result in a smaller substrate. The downstream
decrease in bed material sizes is the result of sorting as a function of
slope. Abrasion accounts for some of the downstream size reduction.
The river contacts the Weber sandstone in the lower portion of the
canyon. In many areas the river flows directly on the sandstone bedrock
which is soft and easily eroded. As the Green River confluence
approaches the river slope becomes more mild. Reaches of cobble
substrate have evolved into a riffle-pool sequence in this section. The
r very mild sloped portions have a sand and gravel substrate. In
contrast, the initial twenty-five miles of canyon, are uniformly steep
punctuated by rapids and backwaters formed by side canyon flood events.
The river is characterized in the cobble substrate reach as a wide
bend, point bar stream with a distinct riffle-pool sequence. The wider
bends are the remanents of the meandering paleochannel whose pattern was
partially preserved in the structure of incised bedrock canyon. The
Yampa riffle-pool sequence is not typical of a natural meandering river
because of the bedrock controls. Riffle spacing is normally five to
seven times the channel width, but riffles have developed in the Yampa
wherever channel expansions have occurred; most often just upstream of
wide bends.
The long term development of the shape, size and orientation of
cobble bars and islands in this reach has been a gradual process
starting with initial formation of meander incision. Over geologic time
a sequence of large, infrequent discharges contribute to the progressive
downstream movement of the cobbles. The riffle-pool sequence evolves as
a function of the large discharges; the cobbles tending to pile up in
the flow expansions upstream of bends. The riffle is initiated by the
leading edge of the depositional region. In this fashion, the bar or
island stability has been established on a quasi-permanent scale with
lesser discharge events inciting limited cobble motion. Without
entrainment, cobble movement is limited to short hops, leaps and rolls,
and the effect on the bar shape is negligible.