No abstract

Nearly instantaneous melting of snow and ice by the March 19, 1982, eruption of Mount St. Helens released a 4 x 10 6 m 3 flood of water from the crater that was converted to a lahar (volcanic debris flow) through erosion and incorporation of sediment by the time it reached the base of the volcano. Over the next 81 km that it traveled down the Toutle River, the flood wave was progressively diluted through several mechanisms. A transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater, when the total sediment concentration had decreased to about 78% by weight (57% by volume). The hyperconcentrated lahar-runout flood wave, transporting immense quantities of sand in suspension, continued to experience progressive downstream dilution. Although turbulence was significantly dampened by the extremely high suspended load, very large standing waves and antidune waves were observed. The hyperconcentra. ted lahar-runout flow deposited an unusual, faintly stratified, coarse sand which locally contained small, isolated gravel lenses. Very similar deposits in .the Quaternary stratigraphy of Moun•t St. Helens and other Cascades volcanoes suggest that lahars may be more frequent than previously recognized. INTRODUCTION Lahar Generation and Transformation On the evening of March 19, 1982, an explosive eruption of Mount St. Helens, Washington [Waitt et al., 1983] rapidly melted about 107 m 3 of snow and ice in the confined southern end of the crater. Subse.quently, 4 x 10 6 m 3 of nearly clear water flowed out of the crater breach as two flood surges and entered a steep and unstable gully system on the north flank of the volcano. Both surges eroded and incorporated enough volcanic debris to emerge 2 km downstream as lahars (volcanic debris flows). The lahars then flowed 27 km down the North Fork Toutle River (Figure 1), overtaking the streamflow in the channel and becoming progressively more dilute with distance downstream. Flo TM transformations occurred within each surge between 27 and43 km downstream from the crater; the evidence for this was a couplet of coarse sedimentary facies deposited by each surge within this reach. The deposits were not typical of either debris flows or normal floods. Downstream of this transition reach the two surges merged and continued as a single flood wav e of hyperconcentrated streamflow, leaving behind an unusual type of fluvial deposit. Sediment concentration continued to decrease with distance downstream. The flood wave reached the Cowlitz River confluence (Figure 1), 83 km downstream from the crater, in approximately 5 hours. Flood wave celerity averaged 4.6 m/s and slowed with distance downstream. Mixing with the Cowlitz River at the confluence reduced peak sediment concentration to Well below hyperconcentrated levels. Rheological Definitions of the Flood "Fluids" The rheological behavior of sediment-water mixtures can be either Newtonian or non-Newtonian, depending on Sediment concentration, sediment type, and particle size distribut...

Periodicals Earthquakes & Volcanoes (issued bimonthly). Preliminary Determination of Epicenters (issued monthly). Technical Books and Reports Professional Papers are mainly comprehensive scientific reports of wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of topographic, hydrologic, and geologic investigations. They also include collections of related papers addressing different aspects of a single scientific topic. Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of resource studies and of geologic and topographic investigations; as well as collections of short papers related to a specific topic. Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest to professional geologists, hydrologists, and engineers. The series covers investigations in all phases of hydrology, including hydrogeology, availability of water, quality of water, and use of water. Circulars present administrative information or important scientific information of wide popular interest in a format designed for distribution at no cost to the public. Information is usually of short-term interest. Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS publications, and they are also available for public inspection at depositories indicated in USGS catalogs. Open-File Reports include unpublished manuscript reports, maps, and other material that are made available for public consultation at depositories. They are a nonpermanent form of publication that may be cited in other publications as sources of information. Maps Geologic Quadrangle Maps are multicolor geologic maps on topographic bases in 71/2-or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure and columnar sections only. Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using geophysical techniques, such as gravity, magnetic, seismic, or radioactivity, which reflect subsurface structures that are of economic or geologic significance. Many maps include correlations with the geology. Miscellaneous Investigations Series Maps are on planimetric or topographic bases of regular and irregular areas at various scales; they present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases which show geology as interpreted from aerial photographs. Series also includes maps of Mars and the Moon. Coal Investigations Maps are geologic maps on topographic or ...

Communities in lowlands near volcanoes are vulnerable to significant volcanic flow hazards in addition to those associated directly with eruptions. The largest such risk is from debris flows beginning as volcanic landslides, with the potential to travel over 100 kilometers. Stratovolcanic edifices commonly are hydrothermal aquifers composed of unstable, altered rock forming steep slopes at high altitudes, and the terrain surrounding them is commonly mantled by readily mobilized, weathered airfall and ashflow deposits. We propose that volcano hazard assessments integrate the potential for unanticipated debris flows with, at active volcanoes, the greater but more predictable potential of magmatically triggered flows. This proposal reinforces the already powerful arguments for minimizing populations in potential flow pathways below both active and selected inactive volcanoes. It also addresses the potential for volcano flank collapse to occur with instability early in a magmatic episode, as well as the "false-alarm lu.s. Geological Survey,

No abstract