Rapid River
General Information
Rapid River is a tributary to the Little Salmon River near Riggins, Idaho. The
study reach is a 1,200 ft length of stream about 2 miles upstream from the confluence
with the Little Salmon River and just upstream of the fish hatchery. The site is on land
administered by the Forest Service at an elevation of about 2,200 ft. The drainage area is
108 mi2 and the geology of the watershed is predominantly sedimentary.
This site is associated with an existing Forest Service gaging station. Streamflow
records and sediment transport measurements are available from water year 1986 to
1999. Additional measurements included a survey of the stream reach, pebble counts of
the substrate surface, core samples of the substrate subsurface material and transport
distance of painted rocks during the high snowmelt flows in 1995. Figure 1 shows a
photograph of looking downstream from the gaging station. There is a cableway just
upstream of the gaging station to facilitate discharge and sediment transport
measurements during high flows.
Figure 1. Rapid River looking downstream from the measurement site.
Streamflow was recorded for water years 1986 through 1999. Estimated average
annual streamflow (Qa) for the stream is 162 ft3/s and the estimated bankfull discharge
(Qb) is 626 ft3/s. Recorded stream discharge for this period ranged from 19.1 ft3/s to
1,357 ft3/s. The largest instantaneous discharge recorded was 1,450 ft3/s on May 31,
1986.
Channel Profile and Cross-Section
Figure 2 shows the longitudinal profile for the channel bed in the center of the
channel, the water surface elevations along each bank at the time of the survey and
bankfull flow elevations (floodplains). The average gradient for the study reach is 0.0108
ft/ft. Cross-sections of the channel were surveyed at seven locations. Discharge and
sediment transport measurements were made at cross-section 1 (XS1) at the cableway
and the movement of painted rocks during the spring and early summer snowmelt flows
in 1995 were measured at two transects about 49 ft and 125 ft upstream of XS3.
170
Rapid River
Elevation, ft
165
160
155
XS4
XS6
XS3
150
XS5
XS2
Water Surface Elevation Left Edge
Water Surface Elevation Right Edge
Center ofChannel Bed Elevation
Bankfull Elevation Right Side
145
XS1
140
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
Distance, ft
Figure 2. Longitudinal profile of the study reach in Rapid River.
Elevation, ft
70
60
Rapid River
50
At Cableway
40
30
20
10
0
Qa
1998 survey
-10
0
50
100
150
200
250
300
350
Distance, ft
Figure 3. Cross-section of Rapid River at the cableway site.
400
450
500
Channel Geometry
Figure 3 shows the cross-section at the sediment transport measurement site,
cross-section 1. The channel geometry relationships for this cross-section are shown in
Figure 4. All data collected during discharge measurements in 1986 through 1999 were
used to develop the displayed power relationships with discharge (Figure 4). Over the
range of discharges when sediment transport was measured (32.3 to 1,300 ft3/s) estimated
stream width, estimated average depth and estimated average velocity varied from 42.2 to
65.9 ft, 0.72 to 2.74 ft, and 1.1 to 7.2 ft/s, respectively. The average reach gradient is
0.0108 ft/ft.
10
100
Average Velocity
0.516
y = 0.178x
2
R = 0.98
Top Width
0.121
y = 27.767x
2
R = 0.81
10
1
Average Depth
0.363
y = 0.203x
2
R = 0.94
0.1
10
100
1000
Average Velocity, ft/s
Width and Depth, ft
Rapid River
1
10000
Discharge, ft3/s
Figure 4. Width, average depth and average velocity versus stream discharge at
the measurement cross section on Rapid River.
Channel Material
A surface pebble count was made at cross-section 1 in July 1994. In October
1995 additional surface pebble counts were made at four cross-sections and four cores of
subsurface material were collected. The average D50 and D90 for the 1994 pebble counts
were 94 mm and 181 mm, respectively. The average D50 and D90 for the surface material
in the reach in 1995 were 63 mm and 220 mm, respectively (Figure 5). The average D50
and D90 for the subsurface material (1995) were 16 mm and 113 mm, respectively.
About 10% of the surface material is sand (2 mm) size or smaller. The particle size
distribution of the floodplain surface strata was quite variable. The median diameter of
the floodplain subsurface strata material was smaller than 0.25 mm.
100
Rapid River
90
80
Percent Finer
70
60
50
Floodplain
Samples
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
and for floodplain material samples collected in Rapid River.
Sediment Transport
The bedload and suspended load measurements in water years 1986 through 2000
were all made in the vicinity of the cableway. The sediment transport data includes 191
measurements of bedload transport and 85 measurements of suspended sediment.
Sediment transport measurements spanned a range of stream discharges from 32.3 ft3/s
(0.20Qa; 0.05Qb) to 1,300 ft3/s (8.02Qa; 2.08Qb). Bedload transport ranged from 0.00270
to 465 t/d and suspended transport ranged from 0.05 to 1,919 t/d. Over the range of
measured discharges, suspended transport accounts for the majority of the material in
transport with approximately four fold difference at the lowest discharge and over an
order of magnitude difference at the highest discharge (Figure 6).
10000
Rapid River
Sediment Transport, tons/day
1000
100
10
Suspended
2.533
y = 4.587E-06x
2
R = 0.89
BCF=1.340
1
Total Bedload
2.233
y = 3.306E-06x
2
R = 0.59
BCF=2.594
0.1
0.01
0.001
Qb
Qa
Suspended
Total Bedload
0.0001
1
10
100
1000
10000
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
usually associated with material in the 0.5 to 2mm diameter size class. At the highest
range of flows material larger than 32mm make up a larger portion of the material in
transport. Twelve of the bedload samples contained material larger than 32 mm diameter
all at discharges equal or greater than 346 ft3/s.
1000
<0.5mm
1.903
y = 4.078E-06x
2
R = 0.69
100
Bedload Rate, tons/day
10
Rapid River
1994 through 2000
0.5-2mm
2.009
y = 6.142E-06x
2
R = 0.69
1
2-8mm
2.081
y = 2.546E-06x
2
R = 0.59
0.1
0.01
8-32mm
1.886
y = 5.962E-06x
2
R = 0.48
0.001
>32mm
2.793
y = 5.656E-08x
2
R = 0.60
<0.5mm
0.5-2mm
2-8mm
Qa
8-32mm
Qb
>32mm
0.0001
1
10
100
1000
Discharge, ft3/s
Figure 7. Bedload transport rate versus discharge for selected size classes
10000
There was a trend of increasing size of the largest particle in the bedload sample
with increasing discharge (Figure 8). The largest particle measured in the bedload
sample was 62 mm at a discharge of 638 ft3/s. There was also a weak trend of increase
median size of the bedload sample with increasing discharge. The largest median size of
a sample was 37.2 mm at a discharge of 597 ft3/s. The information on the largest particle
in the bedload sample and observations of painted rock movement and recently moved
large particles suggest that discharges near bankfull discharge are capable of moving the
median diameter particles on the channel surface.
1000
Rapid River
Largest
Painted
Rock
D50 for four
surface pebble counts
100
Bedload Size, mm
Largest
Particle
1997
R2 = 0.42
D50 for four
subsurface cores
10
R2 = 0.16
1
Qa
Median Size
Qb
Largest Particle
0.1
1
10
100
1000
10000
Discharge, ft3/s
Figure 8. Largest particle in the bedload sample and median size of the sample
versus stream discharge for Rapid River.
Painted Rock Transport
A total of forty painted rocks were placed across two transects (twenty per
transect) on April 18, 1995. Daily mean discharge on the day of rock placement was 127
ft3/s. The size of the rocks ranged from 73 to 180 mm diameter (b-axis) which represents
the D53 up to the D83 of the surface particle size distribution for the reach. Transport of
the rocks as of September 18, 1995 are shown in Figures 9 and 10.
Travel Distance, ft
50
45
Rapid River
40
Painted Rocks
Transect 1
35
The maximum instantaneous
3
discharge of 924 ft /s occurred
on June 17, 1995.
30
25
Particle B-axis, mm
20
15
10
159
5
85
157
81
152 100 145 114 142
M
M
M
M
28
30
32
34
M
M
96
M
M
M
40
42
44
161
99
155
M
95
160
M
89
147
94
149
81
58
60
62
64
M
0
26
36
38
46
48
50
52
54
56
Distance across stream, ft
Figure 9. Transport distance of painted rocks at transect 1.
100
Rapid River
Painted Rocks
90
Travel Distance, ft
80
Transect 2
70
The maximum instantaneous
3
discharge of 924 ft /s
occurred on June 17, 1995.
60
50
40
Particle B-axis, mm
30
20
154
10
85 135
92
132
81 155
90
155
73
M
M
M
M
M
M
M
M
M
31.5
34
36.5
39
41.5
44
46.5
49
51.5
137
75
180 105 135
M
M
M
61.5
64
85 129
102
M
77 140
M
0
29
54
56.5
59
Distance across stream, ft
Figure 10 Transport distance of painted rocks at transect 2.
66.5
69
71.5
74
76.5
Fourteen of the forty rocks were found. Of those found, only two had not moved.
The maximum transport distance was 91 ft for a 137 mm diameter rock and the average
was transport distance was 31.2 ft. The largest daily mean discharge during this period
was 841 ft3/s on June 18, 1995 and the largest instantaneous discharge was 924 ft3/s on
June 17, 1995.