THE EFFECT OF MOUNT ST. HELENS VOLCANISM ON SEDIMENTATION IN THE COWLITZ RIVER, SOUTHWEST WASHINGTON STATE

The syn- and post-event effect of volcanic eruptions on drainage systems in the Washington Cascades is significant to ecosystems, land use, and commerce of the Pacific NW. Preliminary sedimentary, stratigraphic, and tephrochronologic findings indicate that late Quaternary eruptive episodes at Mount St. Helens (MSH) have mobilized sufficient volumes of detritus down the Toutle River to temporarily block the Cowlitz River, a major southwestern Washington drainage. Deposits along the Cowlitz River valley near the Toutle River confluence show the influx of several episodes of increased sedimentation that probably range in age from the Ape Canyon eruptive stage (c.a. 50 ka) to the 1980 eruptions. An exposure upstream of the Toutle-Cowlitz confluence shows three closely spaced debris flow deposits that flowed up the Cowlitz River more than 2 km. The lower two deposits in this exposure consist primarily of poorly sorted sand and matrix-supported gravel with massive and coarse tail normal grading. Pumice contained in these deposits geochemically correlates to the MSH set Jy tephra of the Swift Creek eruptive stage, 10,700 14C years BP. The stratigraphic and topographic position of these deposits, however, suggests correlation with the Pine Creek eruptive period of 2,500 – 3,000 14C years BP, implying that the set Jy tephra and enclosing sediment were remobilized at this time. These debris flow deposits are overlain by up to 35 cm of horizontally laminated sand and silt that accumulated in ponded water of the blocked Cowlitz River. Subsequent burial of this fine-grained sediment beneath pumiceous sand and gravel deposited during the MSH Kalama eruptive period (tephra set Wn, 1480 AD) is evidence for another pulse of volcanogenic sediment into the Cowlitz River via the Toutle River. Continuing research on these Cowlitz River deposits is providing a more complete picture of the impact that future eruptive events at MSH could have on this river system.