River sedimentation caused by the May 18, 1980, eruption of Mount St. Helens, Washington, has been monitored in a continuing program by the U.S. Geological Survey. In this report, sediment transport in streams near Mount St. Helens is summarized from data collected at stream-gaging stations between 1980 and 1990. Sediment-transport monitoring began in earnest on May 18, 1980, on the Toutle River after the north face of Mount St. Helens collapsed into the upper valley of the North Fork Toutle River as an immense debris avalanche (fig. 1). The eruption blast deforested and scorched the terrain in its path while depositing a water-resistant layer of ash and blast material. Volcanic debris flows (popularly called mudflows) transported several million tons of sediment along existing stream channels (Dinehart and others, 1981; Lipman and Mullineaux, 1981). The debris flows from the eruption were as viscous as mortar and littered with timber (fig. 2). Streams in the Toutle, the Cowlitz, and the Lewis River drainage basins were rapidly inundated by the flows with overwhelming supplies of gravel, sand, and silt. Several cubic miles of emplaced sediment were susceptible to rapid erosion and transport from the affected drainage basins.
The river channel and floodplain of the Toutle River, and parts of the Cowlitz and the Lewis Rivers, were altered by extreme sediment deposition and the removal of protective vegetation. The passage of large ships through the Columbia River between Portland, Oregon, and the Pacific Ocean was temporarily halted by sediment deposited in the shipping lane (Meier and others, 1981). The debris-avalanche deposit in the Toutle River blocked inflow from tributaries to form lakes behind unstable embankments. The newly-formed Coldwater and Castle Lakes and the blocked drainage from Spirit Lake constituted a major hydrologic hazard to downstream communities (Childers and Carpenter, 1985).
Even without the hazards from potential breaching of new lakes, fall and winter rains typical of the Pacific Northwest could cause flooding along the sediment-filled lower Cowlitz River. Two questions immediately concerned residents of southwest Washington State: How much sediment would be moved and deposited during storms? And, how long would it be until the rivers recovered their pre-eruption water quality? Those questions have been answered in part with sediment data collected during the first 11 years following the 1980 eruption.
In its 1980 Yearbook, the U.S. Geological Survey (1981, p. 13) outlined data-collection goals and anticipated the results of sediment-transport research at Mount St. Helens:
"The future Water Resources Division program has a dual purpose: to better understand the hydrologic and geomorphic processes involved in the devastation and recovery of the affected area and to provide sound information for hazard warning and resource planning. The program aims to define pre- and post-eruption conditions and monitor hydrologic changes... [It] is anticipated that there will be better understanding of the longer term effects of sediment transport and mudflows..."
Detailed, repetitive measurements of streamflow and sediment concentration were made during storm flows and moderate discharges. Streamflow measurements were used for flood-level predictions, and suspended-sediment measurements were applied to the planning and evaluation of sediment-control works. Aside from contributing to public safety, sediment-transport monitoring provided scientific information about the behavior of river flow at extreme sediment concentration and stream velocity, and about the recovery of stream water quality.
During 1980 through 1990, more than 70,000 water samples were collected from streams in the Mount St. Helens area and analyzed. Following intense rainstorms on the newly affected basins, flood waves would travel the Toutle River channel as fast as 10 mi/h (Dinehart, 1982). More than a million tons of suspended sediment were transported past some gaging stations in a single day. Analysis of daily sediment discharge over the long term showed that annual sediment discharge from the Toutle River basin decreased by a factor of about 20 between 1982 and 1990. Sediment-discharge data have been incorporated in basin-wide studies of channel geometry to understand the accelerated evolution of drainage systems surrounding Mount St. Helens (Meyer and Janda, 1986; citations in Manson and others, 1987; Simon, 1997).
As sediment data were first collected and evaluated, standard field methods were modified to cope with extreme transport rates. Improvements made in automatic sediment sampling and cableway operations increased the accuracy of concentration and discharge measurements. The transition from sandy to gravelly streambeds prompted the design of new bedload samplers. Therefore, analyses of sediment transport are presented together with descriptions of data collection and analysis methods. This report shows that field methods were continually modified to document the evolving river conditions near Mount St. Helens.
This review of sediment transport at gaging stations near Mount St. Helens was prepared to:
This report presents scientific goals and objectives for studies of sediment transport in streams near Mount St. Helens. Principal field observations and developments in methods of data collection and analysis are described in chronological order for the period 1980 to 1990. Sequences of selected sediment data from gaging stations illustrate sediment transport during storm flows. The changing range of sediment variables (sediment-discharge totals, suspended-sediment concentration and particle size, bed-material and bedload size) is identified with data accumulated over the 11-year period.
Most sediment-discharge data were collected at gaging stations far from the sediment sources. Although sediment discharge from the affected basins could be measured, more detailed questions about changing geomorphology of the volcanic areas could not be well answered. Geomorphic adjustments in the study area are discussed in this report where they provide background for sediment-transport processes.
Summaries are provided of lahar behavior and sediment-transport processes as determined from monitoring at gaging stations. References are given for the hydraulics of sediment transport described in original research. Examples of existing and potential data for further research in sedimentation, both regional and basic, are presented as benefits of constant monitoring. By contrasting the data requirements of research with existing data, additional needs in sediment-data collection are described.
In this report, most sediment data are shown in graphical form. Basic sediment data used in this report are available from annual reports of water data for the state of Washington (U.S. Geological Survey, 1980-90), from open-file reports that present sediment data from Mount St. Helens (Dinehart and others, 1981; Dinehart, 1986, 1992b), and on a computer diskette prepared for this report (see Availability of Data).
The study area includes westward-draining streams in Washington State that were affected by the May 18, 1980, eruption of Mount St. Helens (the 1980 eruption, in this report). The Toutle, the Cowlitz, and the Lewis Rivers originate in the Cascades Range of the Pacific Northwest and flow to the Columbia River in southwest Washington State ( fig. 3 ). Rainfall of marine origin supports dense forests along the central and western-facing slopes of the Cascades Range. Mean annual precipitation in the Mount St. Helens area ranges from 46 in/yr at Longview, Washington, to 60 in/yr at Kid Valley, Washington, to nearly 100 in/yr at Spirit Lake (Uhrich, 1990). The drainage area of the Toutle River is 512 mi2, and the combined drainage area of the Muddy River and Pine Creek in the upper Lewis River basin is about 162 mi2. Average discharge of the Toutle River at Tower Road is 2,020 ft3/s; at the Muddy River below Clear Creek, the average discharge is 859 ft3/s (U.S. Geological Survey, 1990).
Elevation in the Mount St. Helens area ranges from 8,365 ft at the present summit of Mount St. Helens to less than 10 ft above sea level at the mouth of the Cowlitz River. The debris avalanche and volcanic blast of the 1980 eruption devastated a 232-mi2 area north of the mountain, and the eruption reduced the mountain's elevation from 9,677 ft (Lipman and Mullineaux, 1981). Hydrologic and sedimentologic effects of the 1980 eruption were described extensively in a report edited by Lipman and Mullineaux (1981). Sedimentation in the Toutle River system through water year 1983 was analyzed by Meyer and Janda (1986). Several research analyses of geomorphic trends in the study area are available, such as Meyer and Martinson (1989) and Simon (1997).
