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

If t*c is 0.03 (Neill, 1968), then for the average water-surface slope
(S=0.00069), and the minimum observed mean depth (D=80, see table 7 in the
Supplemental Data section at the end of this report), do=7.69 mm. Sediment
particles of this size on the bed are at the threshold ~of movement, and all
smaller material is mobile under these flow conditions. The median grain size
(d50) of bed material in the study reach (table 2) is considerably smaller
than do (7.69 mm) for this flow depth. The average size of bed material at
the 95th percentile (des=3.I3 mm) also is smaller than dc, indicating that the
majority of material composing the streambed at Deerlodge Park is a size
capable of transport at the lowest flows.
No obvious nonlinear trends were detected in the sediment discharge and
water discharge data; therefore, sediment transport equations describing the
variation in measured sediment discharge as a function of the water discharge
were derived by a least-squares linear regression of the log-transformed
values. Equations were determined separately for total sediment discharge,
suspended-sediment discharge, bedload discharge, and for different size
fractions of sediment: coarser than 0.062 mm (sand and gravel), finer than
0.062 mm (silt and clay), 0.062 to 0.25 mm, 0.25 to 1.0 mm, and coarser than
1.0 mm (table 3). Based on values of the coefficient of determination (R2),
and standard error of estimate (SE), linear-regression analysis was adequate
to account for most of the variance in data of four of the eight sediment
categories: total sediment discharge, suspended sediment discharge, discharge
of material coarser than 0.062 mm (sand and gravel), and discharge of material
0.062 to 0.25 mm (fine sand). Variance of the data in other sediment catego-
ries was poorly accounted for by linear-regression analysis.
The regression equation for silt and clay-size material (finer than 0.062
mm) in table 3 did not predict well the discharge of this material. The
proportion of silt and clay in the suspended sediment discharge varied consid-
erably (see table 8 in the Supplemental Data section at the end of this
report). 1Jhile the amount of sand transported in suspension varies in phase
with discharge, the amount of silt and clay transported is controlled by the
supply of these fine materials to the stream. The supply of fine material to
a stream reach is influenced by seasonality, the size and duration of runoff
generating storms, and the lag effect of the downstream travel of sediment and
water waves (Richards, 1982). Regression equations for bedload discharge, for
discharge of material in the 0.25 to 1.0 mm range, and for material coarser
than 1.0 mm also did not predict well. Transport of coarse sand and gravel is
strongly discharge dependent. Variance in these data is large because the
sediment discharges of these size ranges were derived from measured bedload
discharges, which also had large variances. The variance in measured bedload
discharge may be large due to a number of reasons, including temporal vari-
ability in bedload discharge which was not accounted for in the sampling
procedures (D. W. Hubbell, U. S. Geological Survey, oral commun., 1984).
Seasonal differences in the supply of sediment to a stream or in water
temperature may affect sediment discharge. Some rivers, for example, the San
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