South Fork Payette River
General Information
The study reach is about a 1,320 ft length of river at the Geological Survey
(USGS) gage 13235000 (South Fork Payette River at Lowman, Id.) in the Boise National
Forest. The stream gaging station is approximately 0.5 miles downstream from Lowman,
Idaho. The site is on land administered by the Forest Service at an elevation of about
3,790 ft. The drainage area upstream of this location is 449.3 mi2 and the geology of the
watershed is predominantly intrusive igneous.
Sediment transport measurements were made by USGS personnel predominantly
during the spring snowmelt discharges of 1994 and 1995. Sediment transport was also
measured on May 17, 1997 at a discharge that exceeded those of the previous two
sampling years. Additional information collected at the site includes a survey of the
stream reach, pebble counts of the surface substrate, core samples of the subsurface
substrate material and samples of floodplain deposits. Figures 1 and 2 show photographs
of the site looking upstream and downstream from the gaging station.
Figure 1. South Fork Payette River looking downstream from the USGS gaging station.
Figure 2. South Fork Payette River looking upstream from the USGS gaging station.
Stream discharge has been recorded since May 1942. Average annual discharge
(Qa) for the period of record is 862 ft3/s and bankfull discharge (Qb) is estimated at 3,050
ft3/s. The range of daily mean discharge for the period of record is 130 ft3/s to 8,900 ft3/s.
The highest instantaneous discharge recorded was 8,980 ft3/s on June 16, 1974.
Cross-Section
Figure 3 shows the cross-section at the USGS gaging station. The average
gradient for the study reach is 0.0040 ft/ft. Most sediment transport measurements were
made from a bridge about 600 ft upstream of the gaging station. During lower
discharges, wading measurements were made either 150 ft or 200 ft downstream of the
gaging station.
60
South Fork Payette River
at the USGS Gage
Elevation, ft
50
40
30
20
10
Qa
0
1994 Survey
1998 Survey
-10
-20
-150 -100
-50
0
50
100
150
200
250
300
350
400
450
Distance, ft
Figure 3. Cross-section of the South Fork Payette River at Lowman, ID at the USGS
Gage.
Channel Geometry
The channel geometry relationships for the cross-section about 200 ft downstream
of the gage where most wading sediment samples were collected in 1994 is shown in
Figure 4. Information for 1994 through 1996 within the range of discharges for the
sediment transport measurements were used to develop the power relationships with
discharge (solid data symbols). Over the range of discharges when sediment transport
was measured at this location (721 to 1,250 ft3/s) estimated stream width, estimated
average depth and estimated average velocity varied from 156.0 to 177.9 ft, 1.55 to 1.85
ft, and 3.0 to 3.8 ft/s, respectively. The average reach slope is 0.0040 ft/ft. The channel
geometry relationships for the bridge cross-section are shown in Figure 5. Sediment
transport measurements during higher discharges in 1994 and for all measurements in
1995 and 1997 were made from the bridge (Figure 5).
10
1000
100
Width
0.216
y = 38.103x
2
R = 0.45
10
Average Velocity
0.462
y = 0.142x
2
R = 0.96
Average Velocity, ft/s
Width and Depths, ft
South Fork Payette River
Wading
Average Depth
0.322
y = 0.186x
2
R = 0.72
1
10
100
1000
1
10000
Discharge, ft3/s
Figure 4. Width, average depth, and average velocity versus stream discharge at the
wading cross section on the South Fork Payette River. (Open data symbols
represent all data from 1941 to 1996 from cross-sections 150 ft to 200 ft
below the gage; solid data symbols represent data from 1994 to 1996 used to
develop the displayed power relationships.)
1000
10
100
Top Width
y = 119.57x0.043
R2 = 0.16
Average Velocity, ft/s
Width and Depth, ft
South Fork Payette River
at Bridge
Mean Velocity
y = 0.158x0.447
R2 = 0.97
10
Mean Depth
y = 0.068x0.477
R2 = 0.97
1
100
1000
1
10000
Discharge, ft3/s
Figure 5. Width, average depth, and average velocity versus stream discharge at the
bridge cross section on the South Fork Payette River. (Open data symbols
represent all data from 1971 to 1997 from the bridge cross-section about 600
ft upstream of the gage; solid data symbols represent data from 1994 to
1997 used to develop the displayed power relationships.)
Channel Material
A surface pebble count was made at a cross-section at the USGS gage in August
1994. Surface pebble counts were made at three cross-sections and three cores of
subsurface material were collected, one at each cross-section, in October 1995. The
average D50 and D90 for the surface material in the reach for the 1994 and 1995 samples
are 55 mm and 175 mm and 110 mm and 258 mm, respectively (Figure 6). The average
D50 and D90 for the 1995 subsurface material samples are 20 mm and 102 mm,
respectively. About 8% of the surface material was sand (2 mm) size or smaller for both
the 1994 and 1995 samples. The D50 of the floodplain samples ranged from 0.12 to 0.25
mm.
100
90
South Fork Payette River
80
Percent Finer
70
Floodplain
Samples
60
50
40
30
20
Surface 1995
Subsurface 1995
Surface 1994
10
0
0.01
0.1
1
10
100
1000
Particle Size, mm
Figure 6. Particle size distribution for surface and subsurface material and floodplain
material samples in the South Fork Payette River study reach.
Sediment Transport
Sediment transport measurements made in 1994 and 1997 includes 72
measurements of bedload transport and 37 measurements of suspended sediment
transport. Sediment transport measurements spanned a range of stream discharges from
721 ft3/s (0.83Qa; 0.24Qb) to 6,390 ft3/s (7.33Qa; 2.10Qb). Bedload transport ranged from
4.67 to 981 t/d and suspended transport ranged from 8.23 to 11,900 t/d. Over the range
of measured discharges, suspended transport typically accounts for the majority of the
material in transport with over an order of magnitude greater suspended transport than
bedload transport at the highest discharges and about similar rates between suspended
and bedload transport at the lowest discharges during which sediment transport was
measured (Figure 7).
100000
South Fork Payette River
Sediment Transport, tons/day
10000
1000
Suspended
3.174
y = 7.360E-09x
2
R = 0.90
BCF=1.238
100
10
Suspended
Total Bedload - Wading
Total Bedload -Bridge 6 in.
Total Bedload - Bridge 3 in.
1
100
Qa
Qb
1000
Total Bedload
2.020
y = 1.261E-05x
2
R = 0.70
BCF=1.353
10000
3
Discharge, ft /s
Figure 7. Bedload and suspended load transport rate versus discharge (The power
relationships include all data. Bedload data symbols code the data by sampling
location and size of the bedload sampler.)
The bedload transport rates by size class (Figure 8) shows that the larger rates are
1000
South Fork Payette River
Bedload Rate, tons/day
100
<0.5mm
1.654
y = 3.207E-05x
2
R = 0.63
0.5-2mm
1.406
y = 5.806E-04x
2
R = 0.31
10
2-8mm
2.015
y = 7.583E-07x
2
R = 0.43
1
0.1
8-32mm
2.140
y = 3.559E-07x
2
R = 0.43
>32mm
1.381
y = 4.534E-04x
2
R = 0.42
0.01
100
Qb
Qa
<0.5mm
0.5-2mm
2-8mm
8-32mm
>32mm
1000
10000
3/
Discharge, ft s
associated with material 0.5 to 2 mm diameter.
Figure 8. Bedload transport rate versus discharge for selected size classes.
The largest particle in the bedload sample generally increased with discharge
(Figure 9). The largest particle measured in a bedload sample was 128 mm at a discharge
of 3,630 ft3/s. Although the largest particle size increased with discharge, the D50 for
most bedload samples was less than 2 mm. However, for many of the bedload samples
collected at discharges greater than bankfull discharge, the median diameter of the
samples exceeded 2 mm and eight samples exceeded a D50 of 20 mm. The relationship
between discharge and the largest particle in the bedload sample and the supporting
observation of the largest particle moved in 1997, and associated instantaneous peak
discharge for the respective snowmelt period suggest that some material about the median
size of the surface material is in motion at discharges near the bankfull discharge.
1000
South Fork Payette River
Bedload Size, mm
100
Largest
Particle
1997
D50 for three
surface pebble counts
D50 for three
subsurface cores
R2 = 0.71
10
R2 = 0.17
1
Median Size
Qa
Qb
Largest Particle
0.1
100
1000
10000
3
Discharge, ft /s
Figure 9. Largest particle in the bedload sample and median size of the sample versus
stream discharge for the South Fork Payette River.