South Fork Red River
General Information
The South Fork Red River confluences with the Main Fork Red River at the Red
River Work Center on the Nez Perce National Forest. The study reach is about a 585 ft
length of river beginning about 500 ft 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 38.2 mi2 and the geology of the watershed is predominantly
metamorphic.
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 information collected at the site include a survey of the stream reach,
pebble counts of the substrate surface, core samples of the substrate 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 looking upstream
and downstream from the gaging station.
Figure 1. South Fork Red River looking upstream from the gaging station.
Figure 2. South Fork Red River looking downstream from the gaging station.
Streamflow records are available for water years 1986 through 1999, typically
from the beginning of the spring snowmelt hydrograph into the fall. Estimated average
annual streamflow (Qa) is about 48.5 ft3/s (17.2 in) and bankfull discharge (Qb) is
estimated at 256 ft3/s. During the period of record, daily mean discharges ranged from
5.02 ft3/s to 489 ft3/s. The highest instantaneous discharge recorded was 678 ft3/s on May
17, 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 0.0146
ft/ft. Cross-sections of the channel were surveyed at four locations. The gage is located
at cross-section 2 (XS2). Most wading sediment transport measurements were made at
cross-section 2.
172
South Fork Red River
170
Elevation, ft
168
166
XS4
XS3
164
Water Surface Elevation Left Edge
Center of Channel Bed Elevation
Water Surface Elevation Right Edge
Bankfull Elevation Left Side
Bankfull Elevation Right Side
162
160
XS2
XS1
158
0
100
200
300
400
500
600
700
Distance, ft
Elevation, ft
192
188
South Fork Red River
184
Cross Section 2
180
176
172
168
Qa
164
1995 survey
1998 survey
160
156
-120
-70
-20
30
80
130
Distance, ft
Figure 3. Longitudinal profile of the study reach in South Fork Red River.
Figure 4. Cross-section 2 of the South Fork Red River.
180
Channel Geometry
Figure 4 shows the cross-section just downstream of the gage, cross-section 2.
The channel geometry relationships for this cross-section are shown in Figure 5 (solid
symbols). During high discharges, measurements are taken from a bridge about 900 ft
downstream of the gage. Data points associated with the bridge cross-section are shown
as open symbols. All data collected in 1986 through 1997 were used to develop the
displayed power relationships with discharge. Over the range of discharges when
sediment transport was measured by wading at cross-section 2 (5.93 to 190 ft3/s)
estimated stream width, estimated average depth and estimated average velocity varied
from 24.4 to 28.7 ft, 0.39 to 1.79 ft, and 0.6 to 3.7 ft/s, respectively. The average reach
gradient is 0.0146 ft/ft.
10
100
Top Width
0.047
y = 22.449x
2
R = 0.53
10
Mean Velocity
0.517
y = 0.245x
2
R = 0.96
1
1
Average Velocity, ft/s
Width and Depth, ft
South Fork Red River
Mean Depth
0.436
y = 0.181x
2
R = 0.92
0.1
1
10
100
0.1
1000
3
Discharge, ft /s
Figure 5. Width, average depth, and average velocity versus stream discharge
at cross-section 2 on the South Fork Red River.
Channel Material
Surface pebble counts were made at one location in July 1994. Surface pebble
counts were also made along three transects in August 1995 and a core of subsurface
material was collected, one per transect. The average D50 and D90 for the 1994 surface
material were 86 mm and 165 mm, respectively (Figure 5). The average D50 and D90 for
the surface material in 1995 were 106 mm and 258 mm, respectively, and the average D50
and D90 for the subsurface material were 25 mm and 145 mm, respectively. About 8% of
the surface material is sand (2 mm) size or smaller. Floodplain samples were collected at
three locations in November 1997. The median size of floodplain surface material was
less than 0.54mm.
100
South Fork
Red River
90
80
Percent Finer
70
60
50
40
Floodplain
Samples
30
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 and subsurface material and
floodplain samples in the South Fork Red River.
1000
Sediment Transport
Sediment transport measurements were made in 1986 through 1999 and include
204 measurements of bedload transport and 136 measurements of suspended sediment.
Sediment transport measurements spanned a range of stream discharges from 5.93 ft3/s
(0.12Qa; 0.02Qb) to 458 ft3/s (9.44Qa; 1.79Qb). Bedload transport ranged from 0.0 to
22.4 t/d and suspended sediment transport ranged from 0.01 to 119 t/d. Over the range of
measured discharges, suspended transport accounts for the majority of the material in
transport with approximately a four to six -fold difference in the rates (Figure 6).
1000
South Fork Red River
Sediment Transport, tons/day
100
Suspended
1.613
y = 1.262E-03x
2
R = 0.68
BCF=1.730
10
1
Total Bedload
1.683
y = 2.374E-04x
2
R = 0.47
BCF=3.460
0.1
0.01
Suspended
Total Bedload
0.001
Qb
Qa
0.0001
1
10
100
Discharge, ft3/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. No curve fitting was done
for sediment >32mm diameter since only two of the samples contained this size class of
material at sampling discharges of 357 and 412 ft3/s.
10
South Fork Red River
1994 through 1999
Bedload Rate, tons/day
1
0.1
<0.5mm
1.526
y=3.234E-05x
2
R = 0.64
0.01
0.5-2mm
1.822
y=4.327E-05x
2
R = 0.60
<0.5mm
0.001
0.0001
2-8mm
2.080
y=4.325E-06x
2
R = 0.58
0.5-2mm
8-32mm
1.512
y=2.792E-05x
2
R = 0.33
>32mm
2-8mm
8-32mm
Qa
Qb
0.00001
1
10
100
1000
Discharge, ft3/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 the bedload sample was 49 mm at a
discharge of 357 ft3/s. The is perhaps a weak trend of increasing median sample diameter
with increasing discharge. The D50 for most samples was <2 mm. 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 bankfull discharge are capable of moving the median diameter particles
on the channel surface.
1000
South Fork Red River
D50 for three
surface pebble
counts
Bedload Size, mm
100
Largest Painted
Rock 1995
Largest Particle
Field Observation
1997
Largest Particles
Bedload Trap
1997
D50 for three
subsurface cores
10
R2 = 0.70
1
R2 = 0.26
Qb
Qa
Median Size
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 South Fork Red River.
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 53.1
ft3/s. The size of the rocks ranged from 70 to 183 mm diameter (b-axis) which represents
the D33 up to the D78 of the surface particle size distribution. Transport of the rocks as of
June 15, 1995 are shown in Figures 9 and 10.
35
South Fork Red River
Transect 1
30
During the painted rock transport
evaluation period, the maximum
instantaneous discharge of 399
3
ft /s occurred on May 6, 1995.
Travel Distance, ft
25
20
15
Particle B-axis, mm
10
180 120 160 109 135
95
130
20
21
72
146
22
23
73 140
83
104
24
26
27
91 140
82
170
89
183
28
30
31
32
33
92
5
0
15
16
17
18
19
25
29
34
Distance across stream, ft
Figure 9. Transport distance of painted rocks at transect 1.
12
South Fork Red River
Transect 2
Travel Distance, ft
10
During the painted rock transport
evaluation period, the maximum
instantaneous discharge of 399
3
ft /s occurred on May 6, 1995.
8
6
Particle B-axis, mm
4
2
150
70
130 105
160 105
157
5.2
6.5
7.7
10.2 11.5 12.7
80
160
78
151
108 136
84
130
112 147
77
144 85
0
9
14
15.2 16.5 17.7
19
20.2 21.5 22.7
24
25.2 26.5 27.7
Distance across stream, ft
Figure 10 Transport distance of painted rocks at transect 2.
29
All of the forty rocks were found and seven had not moved. The maximum
transport distance was 18.3 ft for a 130 mm diameter rock and the average was transport
distance was 3.8 ft. The largest daily mean discharge during the period when painted
rocks were on the bed was 257 ft3/s on May 11, 1995 and the largest instantaneous
discharge was 399 ft3/s.