Morphodynamics of the North Fork Toutle River near Mount St. Helens, Washington
John Pitlick and Erich Mueller
 Geography Department
 University of Colorado-Boulder		 Jon Major and Kurt Spicer
USGS-Cascades Volcano Observatory
Vancouver, WA

NF Toutle bed material
 NF Toutle R
N. Fork Toutle River & Mt. St. Helens
      
Download:

    March, 2009 presentation, CU Boulder (20 Mb)

Problem Statement: 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.  In addition, alluvium stored in terraces that subsequently developed along the NFTR, as well as sediment stored behind a Corps of Engineers retention structure, represent potentially significant long-term sources of sediment.  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, particularly bed load, are poorly constrained, and the information needed to model the future evolution of the system is lacking. 

Near-term Objectives:
  1. Quantify contemporary rates of erosion and sediment transport through the North Fork Toutle River watershed; and
  1. Develop the capability to predict long-term sediment yields.
Approach: Measure slope, bed material grain size and channel geometry at different locations, and use this information along with a regional hydrologic relation to evaluate thresholds for bed load transport and channel widening for different discharges.

nf toutle ~105
 nf toutle ~135


Preliminary Observations and Results:

By coupling the relations for bed material entrainment, flow resistance and continuity with a criterion for bank erosion, we can predict the bankfull width of channels formed by different-sized flows.  The panel on the right shows a comparison between bankfull widths measured in the field and the widths predicted for 2-yr and 20-yr floods.



We can likewise apply a bed load transport relation to calculate instantaneous bed load transport rates for these flows.  The figures below show downstream trends in the total bed load (tons/day) and unit width transport rates (kg/m/s) for 2-yr and 20-yr floods.