GEOLOGIC MAPPING OF THE COLUMBIA MOUNTAIN AREA NEAR HUNGRY HORSE, MONTANA: EMPHASIS ON GEOLOGIC STRUCTURE, SLOPE STABILITY, AND EARTHQUAKE HAZARDS

The Columbia Mountain area lies at the far northern end of the Swan Range, west of Hungry Horse, Montana. Previous geologic mapping has resulted in two contradicting interpretations of the geologic structure. Recent studies done by the Bureau of Reclamation to investigate the seismic hazard for Hungry Horse Dam have inferred a range bounding normal fault along the western front of Columbia Mountain that was not previously identified in past mapping.

A massive landslide, emplaced during deglaciation of the valley due to failure of one of the interfluves on the western front of Columbia Mountain has recently been identified. The scarp of the slide extends 1100m up the mountain where a remnant block clings at the crest. The remnant block is bounded above by large extensional cracks indicating recent movement.

Population growth in the adjacent Flathead Valley has greatly increased over the past fifteen years resulting in housing areas encroaching upon and up the toe of Columbia Mountain. The identification of geologic hazards in the area has raised concerns about the impacts and plausibility of another large magnitude landslide, earthquake, or large rock fall. With a grant from the USGS EDMAP Program, research was conducted during the summer of 2002 by a group of Montana Tech students and faculty, along with staff from the Montana Bureau of Mines and Geology to reconcile the geologic interpretation of the area, identify the range bounding fault using geophysical (gravity and magnetic) surveys, to map the extensional cracks in the remnant block, and to conduct numerical modeling of the remnant block and adjacent slopes to identify their current stability.