Little Buckhorn Creek
General Information
Little Buckhorn Creek is a tributary to Buckhorn Creek which flows into the
South Fork of the Salmon River. The study reach is about a 210 ft length of stream at the
US Geological Survey discontinued gage 13310660 (Little Buckhorn Creek near Krassel
Ranger Station, Idaho), near the confluence with Buckhorn Creek. The site is on land
administered by the Payette National Forest at an elevation of 4,150 ft. The drainage area
is 5.99 mi2 and the geology of the watershed is predominantly intrusive igneous.
Sediment transport measurements were made by Forest Service personnel during
the spring snowmelt flows in 1990 through 1994. Additional measurements at this site
by Case Western Reserve University include a survey of the stream reach and pebble
counts of substrate surface in 1994. In 2000, Forest Service personnel collected
additional information to characterize surface and subsurface channel material. Figure 1
is a photograph of the site looking upstream from a point downstream of the USGS
gaging station.
Figure 1. Little Buckhorn Creek looking upstream.
Streamflow records are available from the USGS for water years 1990 through
1994. The range of daily mean discharge for this period is 1 ft3/s to 34 ft3/s. The highest
daily mean flow occurred on May 31, 1990. Average annual streamflow (Qa) for the
period of record is 4.91 ft3/s and the estimated long-term average annual discharge is 6.65
ft3/s. There were no floodplains identified in this reach, thus, there is no estimate of
bankfull discharge.
Channel Profile and Cross-Section
Figure 2 shows the longitudinal profile for the channel bed in the center of the
channel and the water surface elevations along each bank at the time of the survey. No
floodplains were present to identify the bankfull flow elevations. The average gradient
for the study reach is 0.0509 ft/ft. Cross-sections of the channel were surveyed at three
locations. Discharge and sediment transport measurements were made at or near crosssection 2 (XS2).
168
Little Buckhorn Creek
Elevation, ft
164
XS1
160
XS2
XS3
gage
156
Water Surface Elevation Left Edge
Center of Channel Bed Elevation
Water Surface Elevation Right Edge
152
30
80
130
180
230
Distance, ft
Figure 2. Longitudinal profile of the study reach in Little Buckhorn Creek.
166
Little Buckhorn Creek
Elevation, ft
165
164
163
162
Water Surface
161
160
0
5
10
15
Distance, ft
Figure 3. Cross-section 2 of Little Buckhorn Creek.
20
25
Channel Geometry
Figure 3 shows the cross-section about 15 ft downstream of the gaging station and
the water surface at the time of the survey, July 29, 1994. The station geometry
relationships for this approximate location are shown in Figure 4. Information for 1989
through 1994 within 15 to 20 ft downstream of the gage station (solid symbols) were
used to develop the power relationships with discharge. Data for distances of 25 to 30 ft
downstream of the gaging station (small open symbols) and within 5 ft of the gaging
station (large open symbols) are also shown in Figure 4. Over the range of discharges
when sediment transport was measured (2.4 to 26 ft3/s) estimated stream width, estimated
average depth and estimated average velocity varied from 8.1 to 14.9 ft, 0.35 to 1.12 ft,
and 0.85 to 1.56 ft/s, respectively. The average reach slope is 0.0509 ft/ft.
10
100
Little Buckhorn Creek
10
Average Velocity
0.257
y = 0.677x
2
R = 0.63
1
Average Depth
0.485
y = 0.230x
2
R = 0.93
1
Average Velocity, ft/s
Average Depth or Width, ft
Width
0.258
y = 6.437x
2
R = 0.78
Average Depth
Width
Average Velocity
0.1
1
0.1
100
10
3
Discharge, ft /s
Figure 4. Width, average depth and average velocity versus stream discharge near
cross-section 2 in the study reach of Little Buckhorn Creek.
Channel Material
Surface pebble counts were made near cross-section 2 in July 1994 and at three
cross-sections in September 2000. The D50 and D90 for the 1994 pebble count were 28
mm and 450 mm, respectively (Figure 5) and the D50 and D90 for the 2000 combined
pebble counts were 81 mm and 285 mm, respectively. In 2000, about 12% of the surface
material was sand size or smaller. In September 2000, core samples of the surface and
subsurface material were collected at three locations within the study reach. The D50 and
D90 for the combined surface cores were 119 mm and 191 mm, respectively. The D50 and
D90 for the combined subsurface cores were 14.5 mm and 108 mm, respectively.
100
Little Buckhorn Creek
90
80
Percent Passing
70
60
50
40
30
20
1994 Pebble Counts
2000 Pebble Counts
2000 Surface Cores
2000 Subsurface Cores
10
0
0.1
1
10
100
1000
Particle Diameter, mm
Figure 5. Particle size distribution for surface and subsurface material samples in the
Little Buckhorn Creek study reach.
The sediment transport data include 78 measurements of bedload transport and 73
measurements of suspended sediment transport. Sediment transport measurements
spanned a range of stream discharges from 2.4 ft3/s (0.49Qa) to 26 ft3/s (5.30Qa).
Bedload transport ranged from 0.00190 to 5.02 t/d and suspended transport ranged from
0.0 to 5.75 t/d. At the lowest discharges suspended sediment rate is about an order of
magnitude larger than bedload rates. At the highest discharges suspended and bedload
transport rates are about equal (Figure 6).
100
Little Buckhorn Creek
Sediment Transport, tons/day
10
Suspended
1.362
y = 0.0143x
2
R = 0.41
BCF=1.962
1
Total Bedload
2.271
y = 0.000716x
2
R = 0.50
BCF=1.844
0.1
0.01
Suspended
Total Bedload
Qa
0.001
1
10
100
3
Discharge, ft /sec
Figure 6. Bedload and suspended load transport rate versus discharge.
The bedload transport rates by size class (Figure 7) shows that the larger rates are
associated with material in the 0.5 to 2mm diameter size class. None of the bedload
samples contained material larger than 16 mm diameter.
10
Little Buckhorn Creek
<0.5 mm
1.907
y = 0.000248x
2
R = 0.49
Bedload Tranport, tons/day
1
0.5 - 2 mm
2.212
y = 0.000497x
2
R = 0.49
0.1
2 - 8 mm
2.787
y = 3.953E-05x
2
R = 0.45
0.01
>8 mm
2.242
y = 5.11E-05x
2
R = 0.36
0.001
<0.5mm
0.5-2mm
2-8mm
>8mm
Qa
0.0001
1
10
Discharge, ft3/sec
Figure 7. Bedload transport rate versus discharge for selected size classes.
100
Measurements of the largest particle in the bedload samples were not made for
this site. None of the bedload samples contained particles larger than 16 mm diameter
and 32% of the samples contained particles 8-16 mm in diameter. There was a weak
relationship between the median diameter (D50) of the bedload samples and discharge.
The D50 for all of the samples was sand size, 0.5 to 2.0 mm, or smaller. The largest
median diameter of the bedload samples was 1.9 mm at a discharge of 13 ft3/s.
100
Little Buckhorn Creek
Median Bedload Size, mm
D50 Surface
Material
10
R2 = 0.13
1
Qa
0.1
1
10
100
Discharge, ft3/sec
Figure 8. Median size of the bedload sample versus stream discharge for the
Little Buckhorn Creek study site.