Evaluating River Profile Geometries to Identify Evidence of Active Deformation Associated with the Doty Fault Zone in Southwest Washington
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In 1996, 2007, and 2009, flooding of the Chehalis River near the Town of Chehalis in southwest Washington severely impacted infrastructure and property. Damage was such that Interstate 5, the major transportation throughway in this region, was closed for several days. In 2012, the Washington State Geological Survey and United States Geological Survey began an assessment of the seismic hazards posed by the regional and local geologic systems on proposed construction of a dam near the Town of Pe Ell, Washington. Of these structural systems, the Doty Fault Zone is of interest as; (1) its level of activity is not well known, (2) its geometry is not described in detail, (3) it extends along a portion of the Chehalis River, and (4) would pose a hazard to the construction of the dam if it were active. The Doty Uplift (DU) is one of several basement uplifts in southwest Washington, and the western extent of the Doty Fault Zone bounds the southern boundary of the DU. In this report, the morphology of streams draining the DU are studied and characterized to identify topographic evidence of active deformation associated with the Doty Fault Zone. I performed a digital analysis of the morphology of twenty-two streams, including the description of channel steepness and longitudinal-profile geometry and the identification of knickpoints, or locations of sharp changes in the channel slope along each river. The influence of lithology and the discrimination between discrete (fault-related) and persistent (anticlinal and lithologic) forcings were considered in the analysis. Patterns in the normalized channel steepness index (ksn) across the study site show increasing ksn values moving upstream along drainages within the DU. These range from less than ~150m on the outer margins of the DU to ~400m within it. Anomalous high ksn values that rise to up to 800m point to areas on long profiles that may be identified as knickpoints. Ten knickpoints found within the DU are identified, located between elevations of 137 and 320 meters. They range in size from ~4-meter-deep steps to 36 meter near-vertical drops over distances that range between 6 and 45 meters. Nine of the knickpoints are located at higher elevations that the Doty Fault, yet they do not form similar geometries at similar elevations. In addition, the location of the knickpoints in relation to lithologic boundaries and mass wasting deposits introduces complicating factors that make tying these features to fault-related formation tenuous. Six knickpoints are located within the Crescent Formation, three are located within sedimentary units, and one at a lithologic boundary where streams cross over rocks of variable resistances. The lack of spatial continuity and differences in knickpoint form does not point to the influence of an active Doty Fault. The spatial patterns of normalized channel steepness indices and knickpoints within and around the DU do not point to discrete, fault-offset related causes. Instead, they are likely the product of the structural signature of the Crescent Formation, accented by the influence of lithologic boundaries and mass-wasting deposits. Further investigations of the adjustment of streams of the Doty Hills, thorough in-channel surveys and wider-scale basin instability analyses would help elucidate the topographic evolution of this area.