7087

• UCREFRP Catalog Number: 7087
• Author: Elliot, J. G., J. E. Kircher and P. V. Guerard.
• Title: Sediment Transport in the Lower Yampa River, Northwestern Colorado.
• USFW Year: 1984.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

The pattern of the Yampa River through Dinosaur National Monument is
controlled by the local geology, but the character of the river, its local bed
and bank conditions, and the distribution of riparian vegetation are deter-
mined by the prevailing streamflow and sediment transport regime. Biological
and physical characteristics of this river system are affected by seasonally
controlled streamflows and intermittent transport and storage of sediment.
Much of the sand- and silt-size material introduced into the canyon reach from
Deerlodge Park is mobile at the lowest discharges; however, high discharges,
though relatively infrequent and of short duration, transport a large part of
the total annual sediment load (fig.. 7).
Reduced flood peaks and sediment transport through a river reach can
result in changes in channel geometry, bed-material size, and bank stability.
Schumm (1977, p. 159} summarized the metamorphoses of several rivers in
response to changes in mean annual flood, mean annual discharge, and type of
sediment load. Regulation of flood peaks can have pronounced effects on the
distribution of riparian vegetation as observed on the Green River above the
Yampa River (Potter and others, 1983). Dense vegetation encroaching on a
channel can promote accumulation of sediment and result in channel width
reduction. A reduction in annual streamflow can have a similar effect on
channel geometry, sediment transport, and riparian conditions.
The quantity and composition of sediment carried by a river also are
significant factors affecting river morphology. When the transport of sedi-
ment through a river reach is equal to the amount of sediment supplied to it,
the river is said to be in equilibrium, or graded. Changes in the amount of
sediment supplied to a reach can disrupt equilibrium and lead to aggradation
or degradation of the channel. Sediment surplus (or deficit) equals the
difference between supply to the reach and transport through the reach.
Sediment supply may be reduced progressively because of changes in climate or
land management that result in lower sediment yield, or because sediment is
being stored upstream in the channel or flood plain. More abrupt reductions
in sediment supply occur when sediment is trapped in manmade impoundments.
Depletion of annual streamflow due to transbasin diversions may be accompanied
by a reduction in transport capacity below a diversion structure, resulting in
diminished sediment supplied to downstream reaches.
The annual total sediment load can be reduced significantly if the
magnitude and duration of the highest increments of streamflow are altered,
even though the volume of annual streamflow remains unchanged. Since con-
struction of Flaming Gorge Reservoir on the Green River in Wyoming, the
magnitude and duration of high flows have been reduced, while the duration of
low flows has increased. Annual streamflow below the dam is equal to pre-dam
annual streamflow; but, the annual sediment load measured at a gaging station
located 106 mi downstream has declined by 54 percent (E. D. Andrews, U. S.
Geological Survey, oral common., 1983).
The potential effects of altering the distribution of discharges on sed-
iment transport through Deerlodge Park have been examined for several assumed
flow durations of the Yampa River. The historic duration of streamflows
(curve A, fig. 8) was decreased or increased by percentages equivalent to the
changes observed in the Green River near Jensen, Utah (station 09261000), flow
duration following the construction of Flaming Gorge Reservoir. The assumed
23