Main Fork Red River
General Information
The Main Fork Red River confluences with the South Fork Red River at the Red
River Work Center on the Nez Perce National Forest. The study reach is about a 1000 ft
length of river beginning several hundred feet upstream of the confluence. The site is on
land administered by the Forest Service at an elevation of about 4,350 ft. The drainage
area upstream of this location is 49.7 mi2 and the geology of the watershed is
predominantly metamorphic.
This site is associated with an existing Forest Service gaging station. Streamflow
records measurements are available from water year 1986 to 2000 and sediment transport
measurements are available from water year 1986 to 1999. 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, core samples of floodplain material,
movement of painted rocks during the high snowmelt flows of 1995 and movement of
large bedload into bedload traps during the high snowmelt flows of 1997. Figures 1 and
2 show photographs of the reach.
Figure 1. Main Fork Red River looking upstream from the gaging station.
Figure 2. Main Fork Red River looking downstream from near upper end of the
study reach.
Streamflow records are available for water years 1986 through 2000, typically
from the beginning of the spring snowmelt hydrograph into the fall. Estimated average
annual streamflow (Qa) is about 73 ft3/s (20.0 in) and bankfull discharge (Qb) is estimated
at 330 ft3/s. During the period of record, daily mean discharges ranged from 8.12 ft3/s to
638 ft3/s. The highest instantaneous discharge recorded was 678 ft3/s on May 16, 1996.
Channel Profile and Cross-Section
Figure 3 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 0.0059
ft/ft. Cross-sections of the channel were surveyed at four locations. The USFS gage is
located on the downstream side of a bridge about 115 ft downstream of cross-section 1
(XS1).
168
Elevation, ft
Water Surface Elevation Left Bank
Center of Channel Bed Elevation
Water Surface Elevation Right Bank
Bankfull Elevation Left Side
Bankfull Elevation Right Side
Main Fork Red River
166
164
162
XS4
160
XS3
XS2
158
XS1
156
0
100
200
300
400
500
600
700
800
900
1000
1100
Distance, ft
Figure 3. Longitudinal profile of the study reach in Main Fork Red River.
Elevation, ft
185
180
Main Fork Red River
175
Cross Section 1
170
165
Qa
160
1995 survey
1998 survey
155
150
-50
0
50
100
Distance, ft
Figure 4. Cross-section 1 of the Main Fork Red River.
150
200
Channel Geometry
Figure 3 shows the channel cross-section of the natural channel about 115 ft
upstream of the gaging station. The station geometry relationships for the cross-section
at the gaging station (bridge) are shown in Figure 4. During higher streamflows,
sediment transport is measured from the bridge. During lower flows when wading is
possible, sediment transport measurements were made at several locations, including the
bridge location. Information for 1986 through 1997 were used to develop the power
relationships. Over the range of discharges when sediment transport was measured (9.88
to 646 ft3/s) estimated stream width, estimated average depth and estimated average
velocity vary from 25.6 to 40.7 ft, 0.60 to 2.58, and 0.64 to 6.16 ft/s, respectively. The
average reach slope is 0.0059 ft/ft.
100
10
Main Fork Red River
10
Average Velocity
0.541
y = 0.186x
2
R = 0.97
1
1
Average Velocity, ft/s
Width and Depth, ft
Width
0.111
y = 19.829x
2
R = 0.79
Average Depth
0.349
y = 0.270x
2
R = 0.93
0.1
1
10
100
0.1
1000
3
Discharge, ft /s
Figure 4. Width, average depth, and average velocity versus stream discharge at
the gaging station on the Main Fork Red River.
Channel Material
A surface pebble count was made near cross-section 4 in July 1994. Additional
surface pebble counts were made along four transects in August 1995 and a core of
subsurface material was collected, one per transect. The average D50 and D90 for the
surface material were 68 mm and 110 mm, respectively in 1994 and 50 mm and 160 mm,
in 1995 (Figure 5). The average D50 and D90 for the subsurface material (1995) were 18
mm and 86 mm, respectively. About 24% (1995) of the surface material was sand (2
mm) size or smaller. The median diameter of the surface floodplain material ranged from
0.14 to 0.32 mm.
100
Main Fork
Red 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 and
floodplain samples collected in the Main Fork Red River reach.
Sediment Transport
Sediment transport measurements were made in 1986 through 1999. The
sediment transport data include 200 measurements of bedload transport and 136
measurements of suspended sediment. Sediment transport measurements spanned a
range of stream discharges from 9.88 ft3/s (0.14Qa: 0.03Qb) to 646 ft3/s (8.85Qa: 1.96Qb).
Bedload transport ranged from 0.0 to 23.6 t/d and suspended sediment transport ranged
from 0.02 to 194 t/d. Over the range of measured discharges, suspended transport
accounts for the majority of the material in transport. At the lowest measure discharges
predicted suspended transport is about seven times that of bedload and at the highest
measured discharge about 1.5 times (Figure 6).
1000
Main Fork Red River
Sediment Transport, tons/day
100
10
1
Suspended
1.790
y = 4.902E-04x
2
R = 0.82
BCF=1.363
0.1
0.01
Total Bedload
2.164
y = 2.844E-05x
2
R = 0.70
BCF=2.122
0.001
Suspended
Total Bedload
Qa
Qb
0.0001
1
10
100
3
Discharge, ft /s
Figure 6. Bedload and suspended load transport rate versus discharge.
1000
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 samples
contained material >32mm diameter and associated discharges were greater than 131
ft3/s.
100
Main Fork Red River
1994 through 1999
Bedload Rate, tons/day
10
<0.5mm
1.939
y = 8.041E-06x
2
R = 0.77
1
0.5-2mm
2.381
y = 4.494E-06x
2
R = 0.77
0.1
2-8mm
2.636
y = 3.140E-07x
2
R = 0.66
0.01
<0.5mm
0.5-2mm
2-8mm
8-32mm
>32mm
8-32mm
0.764
y = 8.373E-04x
2
R = 0.10
0.001
>32mm
2.512
y = 2.135E-06x
2
R = 0.54
0.0001
Qa
Qb
0.00001
1
10
100
3/
1000
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 60 mm at a discharge
of 315 ft3/s. Although the largest particle size increased with discharge, the D50 for the
majority of the samples was <2 mm. The largest D50 was 38.9 mm associated with a
sampling discharge of 315 ft3/s. The information on the largest particle in the bedload
sample and observations of painted rock movement, particles caught in bedload traps and
large particles recently moved all suggest that discharges near the bankfull discharge are
capable of moving the median diameter particles on the channel surface.
1000
Main Fork Red River
Largest Painted
Rock 1995
Bedload Size, mm
100
D50 for four
surface pebble
counts
Largest Particle
Field Observation
1997
Largest Particles
Bedload Trap
1997
D50 for four
subsurface cores
10
R2 = 0.53
1
R2 = 0.17
Median Size
Qb
Qa
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 the Main Fork Red River site.
Painted Rock Transport
A total of forty painted rocks were placed across two transects (twenty per
transect) on April 19, 1995. Daily mean discharge on the day of rock placement was 77.6
ft3/s. The size of the rocks ranged from 55 to 150 mm diameter (b-axis) which represents
the D52 up to the D89 of the surface particle size distribution. Transport distance of the
rocks as measured on June 15, 1995 are shown in Figures 9 and 10.
35
Main Fork Red River
Transect 1
30
During the painted rock
transport evaluation period,
the maximum instantaneous
3
discharge of 588 ft /s occurred
on May 6, 1995.
Travel Distance, ft
25
20
15
Particle B-axis, mm
10
5
150 124
88
106
80
72
77
90
75
115 100
96
70
100
70
107
69
82
77
73
3.1
6.4
8.1
9.7 11.5 13.1 14.8 16.4 18.1 19.7 21.5 23.1 24.8 26.4 28.1 29.7 31.5 33.1 34.8
0
4.8
Distance across stream, ft
Figure 9. Transport distance of painted rocks at transect 1.
25
Main Fork Red River
Transect 2
Travel Distance, ft
20
During the painted rock transport
evaluation period, the maximum
instantaneous discharge of 588
3
ft /s occurred on May 6, 1995.
15
10
Particle B-axis, mm
5
75
80
77
107
75
87
80
75
91
3.5
5
6.5
8
9.5
11
12.5
14
15.5
87
55
88
67
91
85
81
83
94
95
110
18.5
20
21.5
23
24.5
26
27.5
29
30.5
32
0
17
Distance across stream, ft
Figure 10 Transport distance of painted rocks at transect 2.
All of the forty rocks were found and only two had not moved. The maximum
transport distance was 23.5 ft for a 87 mm diameter rock and the average was transport
distance was 6.6 ft. The largest daily mean discharge during this period was 420 ft3/s on
May 7, 1995 and the largest instantaneous discharge was 588 ft3/s on May 6.