7930

• UCREFRP Catalog Number: 7930
• Author: Ketcheson, G. L.
• Title: Sediment Rating Equations
• USFW Year: 1986.
• USFW - Doc Type: An Evaluation for Streams in the Idaho Batholith.
• Copyright Material: NO

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DISCHARGE, ft3/s
Figure 4-Sediment rating curves for stream
SC-4 in the Silver Creek study area. The ends
of the curves indicate the limits of the data.
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the statistical problems surrounding small, variable sam-
ple sets. However, if used with other forms of evidence,
a rating equation may provide valuable, additional
documentation.
Sediment Yield Estimation
One of the basic purposes for developing a sediment
rating equation for a stream is to estimate sediment
yield. The sediment rating equation is used to predict an
appropriate sediment rate for periods when no sediment
data are available but streamflow is available or can be
accurately estimated. For sediment yield estimates in
this paper, mean daily streamflow values were used with
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DISCHARGE, ft3/s
Figure 5-Bedload sediment rating curves
for stream SC-2 in the Silver Creek study
area.
the sediment rating equations. Streamflow for the moni-
tored streams on the Boise National Forest was derived
from U.S. Geological Survey gauging stations in the
vicinity. Continuous streamflow records were available
for the streams in Silver Creek.
The two methods used to estimate sediment yield for
streams on the Boise National Forest yielded somewhat
different results (fig. 6). The time-integration technique
typically yielded higher values than the rating equation
approach. A paired t test showed that the difference
between the two methods was significant at the 95 per-
cent level of confidence. Because there was no known
true value for sediment yield from these watersheds, no
evaluation of which estimate was more accurate could be
made.
The time-integration method probably averages higher
because samples taken at or near peak sediment move-
ment are generally averaged over longer periods than
may be appropriate for that level of sediment transport.
Timing of the samples is important because a single
sample must be representative of the prevailing condi-
tions, in some cases for many days. The rating equation
is also affected by the timing. However, the effect may
not be in the same direction or be as significant for the
rating equation because all sample points are analyzed
as a group. Each sample carries more weight in the time-
integration method. Walling (1977) discovered gross
overestimates of total load using sediment rating curves
for three British rivers. However, monthly suspended
loads were both overestimated and underestimated based
on rating equations, with overestimates predominating.
Sediment dams at the mouths of streams in the Silver
Creek study area provided a base sediment yield esti-
mate to compare with estimates from different methods.
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