HOW FLOODING OVER INTERSTATE 5 LED TO REVISION OF THE WASHINGTON GEOLOGICAL SURVEY GEOLOGIC MAPPING PRIORITIES (Invited Presentation)

Floods in 1996, 2007, and 2009 closed Interstate 5 near Chehalis in western Washington for a combined total of 10 days, greatly disrupting commercial travel between Portland, Oregon, and Seattle, Washington. In 2012, then Governor Gregoire convened a working group to develop mitigation measures, one of which was a dam on the upper Chehalis River near Pe Ell in the Willapa Hills. The Washington Geological Survey was tasked with technical review of these plans. Our review of the geotechnical investigation of the proposed dam site led us to observe that the original mapping of the area, published in 1957 and 1958, that went into the state map was done on poor quality topographic maps and before the advent of modern plate tectonic theory. Additionally, lacking any geophysical data, the structural complexity of the older mapping was not well characterized. Since then, high resolution aeromagnetic data and seismic reflection profiling have better characterized faulting and folding in the region. Additionally, recently acquired lidar-based topographic mapping demonstrates that glacial outwash terraces originating from near Mt. Rainier are actually at successively higher elevations going westward up what is now an east-flowing stream, the upper Chehalis River. This suggests that the upper Chehalis is an antecedent stream whose gradient was reversed by uplift of the Willapa Hills, which is the headwaters region for the stream and the location of the proposed dam. Because deformation postdates the oldest glacial deposits from Mt. Rainier, it may be young enough to be a significant seismic source to be considered in the design of the dam. Because studies for design of the dam will need to be completed in the next few years, the Washington Geological Survey has revised its long–term StateMap geologic mapping priorities to investigate the geomorphology and neotectonics of the upper Chehalis basin over the next several years rather than focusing on the northern Puget Lowland.