Morphodynamics of the North Fork Toutle River Near Mount St. Helens, Washington
John Pitlick1, Jon Major2 and Kurt Spicer2
1/Geography Department, University of Colorado, Boulder, CO 80309
2/US Geological Survey Cascades Volcano Observatory, Vancouver, WA 98683
Introduction
Methods, continued
Results, continued
More than 25 years have elapsed since the eruption of Mt. St. Helens, yet the North Fork Toutle River
Hydrologic Data: To estimate channel-forming discharges we developed empirical relations between
Hydraulic Geometry: The downstream adjustments in width and depth of the NFTR scale remarkably
(NFTR) continues to carry probably the highest sediment loads of any river of comparable size in the
high-flows and basin characteristics, using data from 40 gaging stations in SW Washington. Figure 4
well with the mean annual flood (Fig. 6), and the exponents of the resulting power law relations for
conterminous United States [Major et al., 2000]. Much of the sediment carried by the NFTR is derived
shows relations between the mean annual flood and (a) drainage basin area and (b) channel length.
hydraulic geometry are essentially identical to the commonly cited values, B = aQ0.5 and H = cQ0.4.
significant additional long-term sources of sediment.
A
Mean Annual Flood (m 3/s)
Mean Annual Flood (m 3/s)
along the NFTR , as well as sediment stored behind an Army Corps of Engineers retention dam, represent
1000
100
10
Q = 1.00*A 0.87
R2 = 0.62
10
100
1000
A
B
100
10
Q = 0.91*L 1.38
R2 = 0.58
1
1
10.0
100
1
10000
1
10
2
Drainage Area (km )
100
B
10
B = 2.24*Q0.49
R2 = 0.62
10
Length of Main Channel (km)
3
Mean Annual Flood (m /s)
depth and velocity) were determined by simultaneously solving the equations for continuity and flow
approximate threshold for motion increases slightly downstream (Fig. 7a). This effect, coupled with the
resistance, using the measured channel geometry, reach-average slope, S, and surface grain size:
increase in channel width and nearly constant grain size, lead to an almost linear relation between the
where B is width, U is mean velocity, H is depth, u* =
the relation of Parker (1979), using a variable reference Shields stress, *r (Mueller et al. 2005):
avalanche- a total distance of ~25 km. The sites
were located in single-thread reaches (Fig. 2) where
3
5
0.06
Average Gradient, S
0.05
0.03
0.02
D50
0.06
10
2
3
4
5
6
0.02
Distance (km)
7
8
9
10
11
12
1
2
3
4
5
6
7
8
9
10
11
12
Site
25
0.0
5.0
10.0
15.0
20.0
25.0
30.0
Distance (m)
Figure 3. Cross section of the dominant channel.
1.2
D
Width
Bankfull Depth, H (m)
C
Bankfull Width, B (m)
97.5
25
3
30
10
100
3
Discharge (m /s)
Figure 7. Downstream trends in (a) Shields stress and (b) instantaneous bed load transport rate.
20
15
10
5
Depth
1.0
2
3
4
5
6
7
Site
8
9
10
11
12
Field work for this project was completed while the first author was on sabbatical at the US Geological
Survey-Cascades Volcano Observatory; very little of the field work could have been completed without
the advice and logistical support of CVO staff. We would also like to acknowledge the people who
assisted us in the field, including Tom Hale, Dennis Saunders, and especially Rebecca Thomas.
References
0.8
0.6
0.4
1
in slope, width and depth are consistent with the theory that a channel with unlimited sediment supply
Acknowledgements
99.5
measured with a gravelometer, or in the case of large
20
0.04
Site
100.0
98.0
15
and no constraints on width will shape itself to maintain a constant sediment concentration, Qs Q.
0.00
NF 300
Elevation (m)
B
Slope
1
98.5
0.01
high flows. The field data and transport estimates presented here suggest that downstream adjustments
0.08
0.00
99.0
Qb = 0.0015*Q1.05
R2 = 0.33
The North Fork Toutle River has the opportunity to reshape its channel almost every year in response to
100.5
transects on exposed gravel bars. Rock sizes were
grain size distribution
0.10
*
r
change in depth below site 9, where two tributaries- Coldwater Creek and Castle Creek- join the NFTR.
Figure 2. Study site along the N. Fork Toutle River
The grain size distribution of the surface bed
in the point counts, but eliminated from the overall
  2.18S  0.021
0.20
0.10
0.00
A
distances of 100-300 m using the same equipment.
boulders with a measuring tape. Sand was included


tau*ref
0.30
B
Conclusions
and stadia rod; average gradients were surveyed over
particles selected at evenly spaced intervals along

* 4.5
r
tau*
The channel width and depth likewise increase downstream (Fig. 5c and 5d), however, there is a sharp
we could identify a dominant channel with a well-
material was determined from point counts of 300
*


 11.2
*
A
is the shear velocity (all with respect to
the mean annual flood), and ks is the equivalent roughness (3D84). Transport rates were calculated with
Median Grain Size, D50 (m)
Mount St. Helens to the toe of the 1980 debris
cross sections at each site using an engineering level
(gHS)1/2
0.40
Bed Load Transport Rate (m /s)
Q  BHU
no obvious knickpoints, Fig. 5a), whereas there is little change in the median grain size, D50 (Fig. 5b).
at 12 sites, spaced 2-4 km apart from the base of
defined cross-section (Fig. 3). We surveyed three
instantaneous bed load transport rate and discharge corresponding to the mean annual flood (Fig. 7b).
U
11H
 2.5 ln
u*
ks
Channel Geometry and Grain Size: The slope of the NFTR decreases systematically downstream (with
reach-average gradient, and bed material grain size
Mean Annual Flood (m /s)
that the difference between the Shields stress at the mean annual flood and the Shields stress at the
Results
Field Data: We measured channel characteristics,
100
mean annual flood, and calculated the bed load transport rate for that discharge. Flow conditions (width,
q
Methods
3
Figure 6. Downstream hydraulic geometry relations.
Shields Stress
morphology, and (b) develop preliminary estimates of the bed load sediment yield of the upper NFTR.
10
100
Downstream Trends in Shields Stress and Bed Load Transport Capacity: The figure below left shows
Cowlitz River system for decades to come, however, current rates of erosion and sediment transport are
poster summarizes data obtained in 2006 to (a) assess present-day trends in slope, grain size and channel
H = 0.16*Q0.40
R2 = 0.77
Flow and Sediment-Transport Calculations: At each site, we estimated the discharge corresponding to the
Continued erosion in the headwaters of the NFTR will likely affect downstream reaches of the Toutlepoorly constrained, and the information needed to model the evolution of the system is lacking. This
1.0
0.1
1
1000
Figure 4. Regional relations for mean annual flood as a function of (a) drainage area (b) main channel length.
Figure 1. North Fork Toutle River near Mt. St. Helens, WA
Bankfull Depth (m)
1000
Bankfull Width (m)
from the debris avalanche deposited during the May, 1980 eruption (Fig. 1). Alluvium stored in terraces
1
2
3
4
5
6
7
8
9
10
11
12
Site
Figure 5. Downstream trends in (a) slope, (b) median grain size, (c) channel width and (d) channel depth.
Major, J.J., T.C. Pierson, R.L. Dinehart, and J.E. Costa, 2000, Sediment yield following severe volcanic disturbance- A
two-decade perspective from Mount St. Helens, Geology, v. 28, p. 819-822.
Mueller, E. R., J. Pitlick, and J.M. Nelson, 2005, Variation in the reference Shields stress for bed load transport in
gravel-bed streams and rivers, Water Resources Research, v. 41, W04006, doi:10.1029/2004WR003692
Parker, G., Hydraulic geometry of active gravel rivers, J. Hydraul. Div. Am. Soc. Civ. Eng., 105, 1185-1201, 1979.