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

100
J
E
Z
W
w 10
N
O
W
O
Z
W
a
N
N
}
Q
D
N
Z
O
F
Z
W
2
FS
W
N
O
Q
O
J
C
W
co
.1
.01
DISCHARGE, ft3/S
Figure 3-Sediment rating curves for stream SC-6 in the Silver Creek
study area. The extent of the line indicates the limits of the data.
only. Interestingly, it is not significantly dif erent
from the 1978 curve due to high variability. The
shift in the 1982 suspended sediment curve does
coincide with a statistically significant increase in
sediment yield at the sediment dam. This delayed
sediment yield increase at the mouth of watershed
SC-6 is likely due to sediment storage along the
channel with a subsequent release of the stored
material by greater than normal spring runoff in
1982. In this case, the change was documented
through a shift in the rating equation. However, it
is not likely that the stream would be sampled for
6 years under normal monitoring programs on the
typical forest.
The bedload curves for the stream in watershed SC-6
(fig. 3b) show two distinct groupings: the 1976, 1979,
and 1981 curves appear to be a separate group from the
other three in terms of slope. The only statistical differ-
ences are that the slope for 1979 is different from 1975
and 1982. The 1978 data do not produce a significant
equation. Although the 1981 curve is statistically signifi-
cant, the variance about the equation is large, resulting
in wide error bands. This explains why it does not test
significantly different from the other equations. Any
effect the timber harvest had on the bedload rating
equations is not discernible.
During the summer of 1980, a kilometer of road con-
struction within proximity of the stream channel in
watershed SC-4 (102 ha) was completed. The rating equa-
tions appear to shift in response to this (figs. 4a and 4b).
The 1981 rating equations for both suspended (fig. 4a)
and bedload (fig. 4b) sediment are significantly different
from the rest of the years. The 1981 suspended sediment
curve differs only in slope, while the bedload curve
differs both in slope and intercept. The 1982 curves sug-
gest a recovery to near preroad construction conditions,
despite continued sediment delivery to the channel from
the road that year.
Bedload sediment rating equations for the stream in
watershed SC-2 (118 ha) show a significant slope reduc-
tion, between 1979 and 1980 with apparent recovery in
1981 and 1982 (fig. 5). There was no management
activity in this drainage during this time. Suspended
sediment rating curves are not shown because only one
equation from 5 years of record was significant.
Documenting management effects on sediment rating
equations from the two Forests was more difficult
because records of exactly when the activity occurred
were not readily available. The date when the contract
was awarded was available, but the actual operation
may have taken place a year or more later. For this rea-
son, an analysis of Forest data with respect to manage-
ment effects on rating equations is not presented here.
The problems encountered above may be due more to
the data than to rating equations in general. However,
the data used in the above analyses are typical of sedi-
ment data collected on National Forests in the Inter-
mountain West. Shifts in sediment rating curves based
on small sample sizes may result from differing ranges
of streamflow over which data are collected from year to
year. This is somewhat unavoidable depending on the
range of flows present each year. However, sampling
should encompass the same range of flows as much as
possible each year. Otherwise, being on a different por-
tion of the logarithmic curve could provide a different
equation. The change in curves may also result when
sample variance is high and small sample sizes do not
adequately sample the variance. A third reason for
change in curves is a real change in the sediment-
discharge relationship.
Using sediment rating equations as the sole evidence
for management impacts is not advised. The effects of
sample size and sampling scheme on the equations may
not be completely separable from the effects of manage-
ment on the sediment-discharge relationship. Few if any
forests will be able to collect enough samples to alleviate
.1 1 10
.1 1 10