Big Wood River
General Information
The Big Wood River flows out of the southern portion of the Sawtooth National
Recreation Area in central Idaho. The study reach is about a 1,200 ft length of river
about 1.5 miles upstream from the discontinued Geological Survey (USGS) gage
13135500 (Big Wood River near Ketchum) in the Sawtooth National Forest. The site is
approximately 9 miles upstream of Ketchum, Idaho near the Wood River campground on
land administered by the Forest Service. The elevation of the site is about 6,380 ft and
the drainage area is 137.5 mi2 (above the USGS gage). The geology of the watershed is
predominantly mixed volcanic.
Figure 1. Big Wood River at bridge sampling location.
In 1999 and 2000 personnel of Utah State University measured sediment transport
and instantaneous streamflow at this site (Figure 1). Additional information collected at
this site include a survey of the stream reach, pebble counts of the substrate surface and
core samples of the substrate subsurface material.
Streamflow records for complete water years are available for this site 1949
through 1971. The minimum and maximum daily mean discharge for the period of
record are 15 ft3/s and 1,510 ft3/s, respectively. Estimated average annual streamflow
(Qa) is 167 ft3/s and the estimated 1.5 year return interval discharge (Q1.5) is 772 ft3/s.
The maximum discharge recorded was 1,690 ft3/s on May 24, 1967.
Cross-Section
Elevation, ft
Figure 2 shows the cross-section at the sediment transport measurement site at the
bridge shown in Figure 1. The average gradient for the study reach is 0.0091 ft/ft.
8
7
6
5
4
3
2
1
0
-1
-2
Big Wood River
Q1.5
Qa
-5
0
5
10
15
20
25
30
35
40
45
Distance, ft
Figure 2. Cross-section of Big Wood River at the sediment transport measurement site.
Channel Geometry
The channel geometry relationships for this cross-section are shown in Figure 3.
All data collected by USU personnel in 1999 and 2000 were used to develop the
displayed power relationships with discharge. Width is constant at this bridge location
over the measured range of discharge. Over the range of discharges when sediment
transport was measured (213 to 1,090 ft3/s) estimated average depth and estimated
average velocity varied from 1.36 to 3.60 ft, and 3.7 to 7.2 ft/s, respectively. The average
reach gradient is 0.0091 ft/ft.
10
100
Average Velocity
0.403
y = 0.432x
2
R = 0.92
Big Wood River
Average Velocity, ft/s
Width and Depth, ft
Top Width
y = 41.85
10
Average Depth
0.597
y = 0.055x
2
R = 0.96
1
100
1
10000
1000
3
Discharge, ft /s
Figure 3. Width, average depth and average velocity versus stream discharge at
the measurement cross section on the Big Wood River.
Channel Material
A pebble counts were made along three transects in the study reach and three
cores of surface and subsurface material were collected, one at each transect in July 2000.
The D50 and D90 for the combined pebble counts were 119 mm and 353 mm, respectively
(Figure 4). The D50 and D90 for the combined subsurface cores were 25 mm and 152 mm,
respectively, and for the combined surface cores were 155 mm and 210 mm, respectively.
100
Big Wood River
90
80
Percent Finer
70
60
50
40
30
20
2000 Pebble Counts
2000 Subsurface Cores
2000 Surface Cores
10
0
0.1
1
10
100
1000
10000
Particle Size, mm
Figure 4. Particle size distribution for surface and subsurface material samples
in the Big Wood River.
Sediment Transport
Sediment transport measurements made in 1999 and 2000 includes 100
measurements of bedload transport and 26 measurements of suspended sediment
transport. Sediment transport measurements spanned a range of stream discharges from
213 ft3/s (1.28Qa; 0.28Q1.5) to 1,090 ft3/s (6.53Qa; 1.41Q1.5). Bedload transport ranged
from 0.0462 to 371 t/d and suspended transport ranged from 26.0 to 1,400 t/d. Over the
range of discharges when both bedload and suspended load transport rates were
measured, suspended transport accounts for the majority of the material in transport with
over an order of magnitude greater suspended transport than bedload transport at the
lower discharges and about six times as much at the highest discharge (Figure 5).
10000
Big Wood River
Suspended
2.982
y = 4.650E-07x
2
R = 0.82
BCF=1.102
Sediment Transport, tons/day
1000
Total Bedload
3.540
y = 1.534E-09x
2
R = 0.87
BCF=1.300
100
10
1
Qa
0.1
100
Suspended
Total Bedload
Q1.5
1000
Discharge, ft3/s
Figure 5. Bedload and suspended load transport rate versus discharge.
10000
The bedload transport rates by size class (Figure 6) shows that the larger rates are
usually associated with material in the 0.5 to 2mm diameter size class.
1000
Big Wood River
Bedload Rate, tons/day
100
0.5-2mm
3.216
y = 5.283E-09x
2
R = 0.89
<0.5mm
0.5-2mm
2-8mm
8-32mm
>32mm
10
<0.5mm
3.081
y = 2.225E-09x
2
R = 0.92
2-8mm
3.680
y = 1.457E-10x
2
R = 0.84
1
8-32mm
4.311
y = 1.486E-12x
2
R = 0.72
>32mm
1.551
y = 1.804E-04x
2
R = 0.37
0.1
Qa
0.01
100
Q1.5
1000
Discharge, ft3/s
Figure 6. Bedload transport rate versus discharge for selected size classes.
10000
The size of the largest particle in the bedload sample increased with discharge
(Figure 7). The largest particle measured in a bedload sample was 79 mm at a discharge
of 1,000 ft3/s. At discharges greater than the 1.5 year return interval discharge (772 ft3/s)
the largest particle always exceeded 22 mm. There is also a trend of increasing median
size of the bedload sample with increasing discharge. The D50 for most of the bedload
samples collected at discharges less than the 1.5 year return interval discharge were in the
sand size, 0.5 to 2.0 mm. The information on the largest particle in the bedload sample
suggests that discharges somewhat larger than the 1.5-year return interval discharge are
capable of moving the median diameter particles on the channel surface.
1000
Big Wood River near Ketchum
D50 for three
surface pebble counts
100
Bedload Size, mm
R2 = 0.63
D50 for three
subsurface cores
10
R2 = 0.08
1
Qa
Median Size
Largest Particle
Q1.5
0.1
10
100
1000
10000
Discharge, ft3/s
Figure 7. Median size of the bedload sample and the largest particle size versus
stream discharge for the Big Wood River.