North Fork Clearwater River
General Information
The study reach is about a 2,200 ft length of stream at the Geological
Survey(USGS) gage 13340600 (North Fork of the Clearwater River near Canyon Ranger
Station, ID). The site is about 1.7 miles downstream from the Canyon Ranger Station,
Idaho. The site is on land administered by the Forest Service at an elevation of about
1,660 ft above sea level. The drainage area is 1,294.2 mi2 and the geology of the
watershed is intrusive igneous and metamorphic rock.
Sediment transport measurements were made by USGS personnel predominantly
during the spring snowmelt flows of 1994 and 1995. Two additional measurements were
made during the high spring discharges of 1997. Additional measurements at this site
include a survey of the stream reach, pebble counts of the surface substrate and core
samples of the subsurface substrate material. Figures 1 and 2 show photographs of the
site looking upstream and downstream from the bridge, just downstream of the USGS
gaging station. The cableway is approximately 15 ft downstream of the gaging station.
Figure 1. North Fork Clearwater River looking upstream from bridge.
Figure 2. North Fork Clearwater River looking downstream from the bridge.
Streamflow records are available for April 1967 to present. Estimated average
annual streamflow (Qa) for the stream is 3,510 ft3/s and the estimated bankfull discharge
(Qb) is 16,000 ft3/s. Stream discharge was low in 1994 and the largest daily mean
discharge was only 12,600 ft3/s. In 1995 the largest daily mean discharge recorded was
34,200 ft3/s on November 30, well above bankfull discharge. The largest instantaneous
discharge for the period of record was 37,500 ft3/s on November 30, 1995.
Cross-Section and Channel Geometry
Figure 3 shows the cross-section at the sediment transport measurement site, the
cableway about 15 ft downstream of the gaging station. The average gradient for that
section of the study reach from the downstream bridge and extending about 1000 ft
upstream of the gage station is 0.0005 ft/ft. Just upstream of this reach is a steep rapid
that was not included in the gradient calculations.
60
North Fork Clearwater River
Elevation, ft
50
40
30
20
Qa
10
0
1990 Survey
1998 Survey
-10
-20
0
50
100
150
200
250
300
350
400
Distance, ft
Figure 3. Cross-section of the North Fork Clearwater River at the USGS cableway.
The channel geometry relationships for this cross-section are shown in Figure 4.
All data collected in 1994 through 1998 from the cableway were used to develop the
displayed power relationships with discharge. Over the range of discharges when
sediment transport was measured (3,560 to 34,400 ft3/s) estimated stream width,
estimated average depth and estimated average velocity varied from 242 to 295 ft, 5.5 to
100
1000
Top Width
0.087
y = 118.72x
2
R = 0.90
100
10
Average Velocity
0.574
y = 0.025x
2
R = 0.99
1
10
Average Velocity, ft/s
Width and Depths, ft
North Fork Clearwater River
Average Depth
0.339
y = 0.342x
2
R = 0.98
1
100
1000
10000
0.1
100000
3
Discharge, ft /s
11.8 ft, and 2.7 to 9.9 ft/s, respectively. The average reach gradient is 0.0005 ft/ft.
Figure 4. Width, average depth, and velocity versus stream discharge at the cableway
cross section on the North Fork Clearwater River. (Solid symbols represent
data used to develop the power relationship; 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 at the cableway cross-section in September
1994. Surface pebble counts were also made along three transects in September 1995
and three cores were collected of subsurface material. The average D50 and D90 for the
surface material were 60 mm and 334 mm, respectively, in 1994 and 95 mm and 282
mm, respectively, in 1995. The average D50 and D90 for the subsurface material in 1995
were 23 mm and 135 mm, respectively (Figure 5). The median size of floodplain
material was highly variable and the most notable feature is the larger percentage of
material smaller than about 0.5 mm.
100
North Fork Clearwater River
90
80
Percent Finer
70
60
50
40
30
Floodplain
Samples
20
Surface 1995
Subsurface 1995
Surface 1994
10
0
0.01
0.1
1
10
100
Particle Size, mm
Figure 5. Particle size distribution for surface, subsurface and floodplain
material samples in the North Fork Clearwater River.
1000
Sediment Transport
Sediment transport measurements made in 1994 through 1997 includes 72
measurements of bedload transport and 36 measurements of suspended sediment
transport. Sediment transport measurements spanned a range of stream discharges from
3,560 ft3/s (1.01Qa; 0.22Qb) to 34,400 ft3/s (9.80Qa; 2.15Qb). Bedload transport ranged
from 0.140 to 5,920 t/d and suspended transport ranged from 21.1 to 411,000 t/d. Over
the range of measured discharges, suspended transport accounts for the majority of the
material in transport with between one and two orders of magnitude greater suspended
transport than bedload transport at all discharges (Figure 6).
1000000
North Fork Clearwater River
Sediment Transport, tons/day
100000
10000
1000
100
10
1
Suspended
3.707
y = 7.714E-13x
2
R = 0.92
BCF=1.241
Total Bedload
3.862
y = 4.505E-15x
2
R = 0.88
BCF=1.272
Qa
0.1
1000
Qb
Suspended
Total Bedload
10000
Discharge, ft3/s
Figure 6. Bedload and suspended load transport rate versus discharge.
100000
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. Only five of the samples
contained particles greater than 32 mm and these were samples with discharges of 14,700
ft3/s and larger.
10000
Bedload Rate, tons/day
1000
100
North Fork Clearwater River
<0.5mm
3.576
y = 1.368E-14x
2
R = 0.86
10
0.5-2mm
3.596
y = 2.869E-14x
2
R = 0.85
1
2-8mm
3.937
y = 1.864E-16x
2
R = 0.78
8-32mm
4.181
0.1 y = 2.062E-17x
2
R = 0.81
>32mm
0.01
3.059
y = 3.349E-12x
2
R = 0.55
Qa
0.001
1000
Qb
10000
<0.5mm
0.5-2mm
2-8mm
8-32mm
>32mm
100000
3/
Discharge, ft s
Figure 7. Bedload transport rate versus discharge for selected size classes.
The size of the largest particle in the bedload sample increased with discharge
(Figure 8). The largest particle measured in a bedload sample was 74 mm at a discharge
of 33,700 ft3/s. At discharges greater than bankfull discharge (16,000 ft3/s) the largest
particle exceeded 40 mm in all but one instance. The D50 for most of the bedload
samples collected at discharges less than bankfull discharge was in the sand size range,
0.5 to 2 mm. At discharges greater than bankfull discharge the median diameter ranged
from 3.6 to 19.9 mm. The largest median diameter of a bedload sample was 19.9 mm for
a sample collected at 32,800 ft3/s. The information on the largest particle in the bedload
sample and observations of recently moved large rocks and associated instantaneous peak
discharges for the 1997 snowmelt period suggest that the median diameter particles on
the channel surface begin to move at discharges larger than the bankfull discharge.
1000
North Fork Clearwater River
D50 for three
surface pebble
counts
100
Bedload Size, mm
Largest
Particle
1997
R2 = 0.64
D50 for three
subsurface cores
10
R2 = 0.32
1
Qb
Qa
0.1
1000
10000
Median Size
Largest Particle
100000
3
Discharge, ft /s
Figure 8. Median size of the bedload sample and the largest particle size
versus stream discharge for the North Fork Clearwater River.