Selway River
General Information
The study reach is about a 2,340 ft length of river at the USGS gage 13336500
(Selway River near Lowell, ID) in the Nez Perce National Forest. The stream gaging
station is approximately 0.2 miles upstream from O’Hara Creek. The site is on land
administered by the Forest Service at an elevation of about 1,540 ft. The drainage area
upstream of this location is 1,913.1 mi2 and the geology of the watershed is
predominantly intrusive igneous and metamorphic.
Sediment transport measurements were made by USGS personnel predominantly
during the spring snowmelt flows in 1994 and 1995. Additional measurements were
made during a winter flood in December 1995 and near the snowmelt peak flows of 1997.
All measurements were made from a cableway with the exception of the December 1995
measurements which were made from a bridge about 1000 ft downstream of the
cableway. Additional information collected at the site include a survey of the stream
reach, pebble counts of the surface material, core samples of the subsurface material
material and samples of floodplain deposits. Figures 1 and 2 show photographs of the
site looking upstream and downstream from the USGS gaging station. The cableway is
approximately 210 ft downstream of the gage.
Figure 1. Selway River looking upstream from the USGS gaging station.
Figure 2. Selway River looking downstream from the USGS gaging station.
Stream discharge records are available for October 1929 to present. Average
annual discharge (Qa) for the period of record is 3,730 ft3/s and bankfull discharge (Qb) is
estimate at 23,000 ft3/s. The range of daily mean discharge for the period of record is
150 ft3/s to 45,300 ft3/s. The highest instantaneous discharge recorded was 48,900 ft3/s
on May 29, 1948.
Cross-Section
Figure 3 show a representative cross-section about 10.5 ft upstream of the USGS
55
Selway River
Elevation, ft
45
35
25
15
Qa
5
-5
-15
-250
1994 survey
1998 survey
-150
-50
50
150
250
Distance, ft
gage house. The average gradient for the study reach is 0.0021 ft/ft.
Figure 3. Cross-section of the Selway River near Lowell, ID.
350
Channel Geometry
The station geometry relationships for the cross-section at the cableway are
shown in Figure 4. Information for 1994 through 1997 for the sediment transport
measurements taken from the cableway were used to develop the power relationships
with discharge. Over the range of discharges when sediment transport was measured
(4,760 to 37,700 ft3/s) estimated stream width, estimated average depth and estimated
average velocity varied from 279.2 to 297.0 ft, 3.8 to 9.5 ft, and 4.5 to 10.0 ft/s,
respectively. The average reach slope is 0.0021 ft/ft.
1000
100
100
Width
0.030
y = 216.53x
2
R = 0.60
Average Velocity, ft/s
Width and Depth, ft
Selway River
10
Average Velocity
0.580
y = 0.028x
2
R = 0.99
10
1
Average Depth
0.391
y = 0.164x
2
R = 0.98
1
100
1000
10000
0.1
100000
Discharge, ft3/s
Figure 4. Width, average depth, maximum depth and velocity versus stream discharge
at the cableway cross-section on the Selway River. (Solid symbols
represent data used to develop the power relationships; open symbols were
not used because they are either outside the range of discharges when
sediment transport was measured or represent data collected prior to 1994.)
Channel Material
A surface pebble count was made in September 1994 near the USGS gage.
Surface pebble counts were also made at three cross-sections and three cores of
subsurface material were collected, one at each cross-section in September of 1995. The
average D50 and D90 for the surface material were 200 mm and 315 mm, respectively in
1994 and 173 mm and 310 mm, respectively, in 1995 (Figure 5). The average D50 and
D90 for the 1995 subsurface material are 24 mm and 131 mm, respectively. About 11%
of the surface material is sand (2 mm) size or smaller. Three floodplain locations were
sampled in November of 1997. The D50 of all of the floodplain samples was smaller than
0.27 mm.
100
Selway River
90
80
Percent Finer
70
60
Floodplain
Samples
50
40
30
20
Surface 1995
Subsurface 1995
Surface 1994
10
0
0.01
0.1
1
10
100
1000
Particle Size, mm
Figure 5. Particle size distribution for surface and subsurface material samples
collected in the Selway River.
Sediment Transport
The bedload and suspended load measurement made in water years 1994 through
1997 were all made from the cableway, with the exceptions of the December 1995
measurements which were made from a bridge about 1000 downstream of the gage. The
sediment transport data includes 72 measurements of bedload transport and 36
measurements of suspended sediment. Sediment transport measurements spanned a
range of stream discharges from 4,760 ft3/s (1.27Qa; 0.21Qb) to 37,700 ft3/s (10.05Qa;
1.64Qb). Bedload transport ranged from 0.1 to 368 t/d and suspended transport ranged
from 16.6 to 64,300 t/d. Over the range of measured discharges, suspended transport
accounts for the majority of the material in transport with approximately an order of
magnitude greater suspended transport than bedload transport (Figure 6).
100000
Selway River
Sediment Transport, tons/day
10000
1000
Suspended
3.598
y = 6.451E-13x
2
R = 0.76
BCF=1.883
100
10
Total Bedload
4.426
y = 9.437E-18x
2
R = 0.84
BCF=1.291
1
Qa
0.1
1000
Qb
Suspended
Total Bedload
10000
100000
3
Discharge, ft /s
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 0.5 to 2 mm diameter. No power relationship is displayed for
material greater than 32 mm since only two bedload samples contained this size of
material. Bedload samples with material larger than 32 mm diameter were collected at
discharges of 37,000 and 37,700 ft3/s.
1000
Selway River
Bedload Rate, tons/day
100
10
<0.5mm
3.945
y = 1.545E-16x
2
R = 0.81
1
0.5-2mm
4.145
y = 9.036E-17x
2
R = 0.76
0.1
<0.5mm
0.5-2mm
2-8mm
8-32mm
>32mm
0.01
0.001
1000
2-8mm
4.067
y = 1.259E-17x
2
R = 0.76
Qb
Qa
8-32mm
3.124
y = 7.114E-14x
2
R = 0.40
10000
100000
3/
Discharge, ft s
Figure 7. Bedload transport rate versus discharge for selected size classes.
The largest particle in the bedload sample generally increased with discharge
(Figure 8). The largest particle measured in the bedload sample was 82 mm at a
discharge of 37,000 ft3/s. Although the largest particle size increased with discharge, the
D50 for most samples was in the sand size, ranging from 0.48 to 1.12 mm. On two
occasions, associated with higher discharges, the D50 was appreciable larger, 37.6 mm at
a discharge of 37,700 ft3/s and 51.3 mm at a discharge of 37,000 ft3/s. The relationship
between discharge and the largest particle in the bedload sample and supporting
observations of large particles recently moved and associated instantaneous peak
discharges for that snowmelt period suggest that discharges larger than bankfull
discharge are required to initiate transport of material about the median size of the surface
material.
1000
Selway River
Bedload Size, mm
100
Largest Particle
1997
D50 for three
surface pebble counts
D50 for three
subsurface cores
2
R = 0.75
10
R2 = 0.29
1
Qa
Qb
Median Size
Largest Particle
0.1
1000
10000
100000
3
Discharge, ft /s
Figure 8. Largest particle in the bedload sample and median size of the sample
versus stream discharge for the Selway River.