Morphodynamics of the North Fork Toutle River Near Mount St. Helens, Washington

1. 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 Methods, continued Hydrologic Data: To estimate channel-forming discharges we developed empirical relations between high-flows and basin characteristics, using data from 40 gaging stations in SW Washington. Figure 4 shows relations between the mean annual flood and (a) drainage basin area and (b) channel length. Results, continued Hydraulic Geometry: The downstream adjustments in width and depth of the NFTR scale remarkably well with the mean annual flood (Fig. 6), and the exponents of the resulting power law relations for hydraulic geometry are essentially identical to the commonly cited values, B = aQ0.5 and H = cQ0.4. Introduction More than 25 years have elapsed since the eruption of Mt. St. Helens, yet the North Fork Toutle River (NFTR) continues to carry probably the highest sediment loads of any river of comparable size in the conterminous United States [Major et al., 2000]. Much of the sediment carried by the NFTR is derived from the debris avalanche deposited during the May, 1980 eruption (Fig. 1). Alluvium stored in terraces along the NFTR , as well as sediment stored behind an Army Corps of Engineers retention dam, represent significant additional long-term sources of sediment. A B A B Figure 4. Regional relations for mean annual flood as a function of (a) drainage area (b) main channel length. Figure 6. Downstream hydraulic geometry relations. Flow and Sediment-Transport Calculations: At each site, we estimated the discharge corresponding to the mean annual flood, and calculated the bed load transport rate for that discharge. Flow conditions (width, depth and velocity) were determined by simultaneously solving the equations for continuity and flow resistance, using the measured channel geometry, reach-average slope, S, and surface grain size: Downstream Trends in Shields Stress and Bed Load Transport Capacity: The figure below left shows that the difference between the Shields stress at the mean annual flood and the Shields stress at the approximate threshold for motion increases slightly downstream (Fig. 7a). This effect, coupled with the increase in channel width and nearly constant grain size, lead to an almost linear relation between the instantaneous bed load transport rate and discharge corresponding to the mean annual flood (Fig. 7b). Figure 1. North Fork Toutle River near Mt. St. Helens, WA Continued erosion in the headwaters of the NFTR will likely affect downstream reaches of the Toutle-Cowlitz River system for decades to come, however, current rates of erosion and sediment transport are poorly constrained, and the information needed to model the evolution of the system is lacking. This poster summarizes data obtained in 2006 to (a) assess present-day trends in slope, grain size and channel morphology, and (b) develop preliminary estimates of the bed load sediment yield of the upper NFTR. A B where B is width, U is mean velocity, H is depth, u*= (gHS)1/2 is the shear velocity (all with respect to the mean annual flood), and ks is the equivalent roughness (3D84). Transport rates were calculated with the relation of Parker (1979), using a variable reference Shields stress, *r (Mueller et al. 2005): Methods Field Data: We measured channel characteristics, reach-average gradient, and bed material grain size at 12 sites, spaced 2-4 km apart from the base of Mount St. Helens to the toe of the 1980 debris avalanche- a total distance of ~25 km. The sites were located in single-thread reaches (Fig. 2) where we could identify a dominant channel with a well-defined cross-section (Fig. 3). We surveyed three cross sections at each site using an engineering level and stadia rod; average gradients were surveyed over distances of 100-300 m using the same equipment. The grain size distribution of the surface bed material was determined from point counts of 300 particles selected at evenly spaced intervals along transects on exposed gravel bars. Rock sizes were measured with a gravelometer, or in the case of large boulders with a measuring tape. Sand was included in the point counts, but eliminated from the overall grain size distribution Results Channel Geometry and Grain Size: The slope of the NFTR decreases systematically downstream (with no obvious knickpoints, Fig. 5a), whereas there is little change in the median grain size, D50 (Fig. 5b). The channel width and depth likewise increase downstream (Fig. 5c and 5d), however, there is a sharp change in depth below site 9, where two tributaries- Coldwater Creek and Castle Creek- join the NFTR. Figure 7. Downstream trends in (a) Shields stress and (b) instantaneous bed load transport rate. Conclusions The North Fork Toutle River has the opportunity to reshape its channel almost every year in response to high flows. The field data and transport estimates presented here suggest that downstream adjustments in slope, width and depth are consistent with the theory that a channel with unlimited sediment supply and no constraints on width will shape itself to maintain a constant sediment concentration, QsQ. A B Figure 2. Study site along the N. Fork Toutle River • Acknowledgements • 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 • 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. C D Figure 3. Cross section of the dominant channel. Figure 5. Downstream trends in (a) slope, (b) median grain size, (c) channel width and (d) channel depth.