QUATERNARY GEOLOGY OF THE LOWER LEWIS RIVER, WA: INFLUENCE OF VOLCANOGENIC SEDIMENTATION FOLLOWING MT. ST. HELEN’S ERUPTIONS

The Lewis River of southwestern Washington is one of many Cascade Range drainages that have impacted the flow of the Columbia River following eruptive events during the Quaternary period. Volcanogenic sediment is transported by the Lewis River from the southeastern flanks of Mount St. Helen’s (MSH) to the Columbia River confluence near Woodland, WA. Examination of more than 30 exposures along the lower Lewis River at elevations <100 m asl downstream of Merwin Dam (~19 river miles) reveal four types of Quaternary deposits: 1) cross-stratified fluvial sand and gravel, 2) massive to cross-stratified pumicious fluvial sand, 3) massive volcanic debris flow deposits, and 4) massive Missoula flood silt.

Stratigraphic and tephrochronologic analysis expose a sedimentary record affected by at least three eruptive events over the past ca. 40,000 years. Volcanogenic sediment mobilized during these eruptions helped generate a large delta that has prograded into the Columbia River. Debris flow deposits correlated to MSH set C in addition to tephra from MSH sets S, P, and W are found intercalated with organic-bearing, traction-dominated fluvial sediment. Slumps, dikes, brecciated layers, and disturbed bedding found in younger fluvial deposits are interpreted as liquefaction features, presumably triggered by a major earthquake in 1700 AD.

Three distinct terraces exist within the lower Lewis River valley. Radiocarbon and tephrochronologic data have permitted correlation of the construction of terraces at 12 m and 17 m asl (<8 m above the modern river) to aggradation during the 1480 eruption of MSH (set W). These terraces are inset into Tertiary rocks and unconsolidated, rhythmically bedded silt from glacial Lake Missoula flooding. The Missoula flood deposits are up to 30 m thick and are the capping deposit on a terrace that lies at 61 m asl and extends upstream just south of Merwin Dam.

The deposits in the lower Lewis River provide a record of both depositional history and volcanic history, in turn, providing a sense of magnitude and frequency of volcanic events affecting the area. Therefore, an understanding of the development of drainage systems in volcanic terrains is necessary in recognizing the effects and hazards future eruptive events could have on both society (e.g. river commerce) and its ecosystem.