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Larry N. Smith Montana Bureau of Mines and Geology Montana Tech

Columbia Mountain Slide:. Latest Pleistocene and Future(?) Failure, Flathead County, Montana. Larry N. Smith Montana Bureau of Mines and Geology Montana Tech. Whitefish Lake. Columbia Falls. Slide. Overview from Columbia Mountain, looking west. Slide. Location. Columbia Falls.

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Larry N. Smith Montana Bureau of Mines and Geology Montana Tech

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  1. Columbia Mountain Slide: Latest Pleistocene and Future(?) Failure, Flathead County, Montana Larry N. SmithMontana Bureau of Mines and GeologyMontana Tech

  2. Whitefish Lake Columbia Falls Slide Overview from Columbia Mountain, looking west

  3. Slide Location Columbia Falls Swan Range Slide

  4. Aerial Photograph Slide

  5. Columbia Mountain Slide - Digital Orthophoto Perspective

  6. Debris Avalanche Rock Slide Scarp

  7. Toe of Slide

  8. Debris in Slide Scale ~ 2 feet

  9. Kettles Rock Slide Scarp Debris Avalanche Kettle Latest Pleistocene Columbia Mountain Slide

  10. Latest Pleistocene Columbia Mountain Slide • Size: • Volume: 30 million yards3 • Area: 0.67 miles2 • Morphometry: • Height: 3,530 feet • Length: 2 miles • Mechanism: • Debris Avalanche Followed by Rock Slide • Trigger: • Deglaciation: Oversteepened, Water-saturated Slope(?) • Removal of Lateral Support

  11. “Free Face” Tension Cracks

  12. Dogs for scale Fracture in Bedrock near Columbia Mountain

  13. Fracture in Bedrock near Columbia Mountain Bed ~ 2 feet thick for scale

  14. Fractures:

  15. Possible Future Failure 37 million yd3 of rock Previous Scarp Tension Cracks Possible Failure Plane

  16. Cross Section W E Fractures Parallel To Possible Failure Plane ) 7 0 0 0 et e 6 0 0 0 (f P r e v i o u s 5 0 0 0 de u 4 0 0 0 A v a l a n c h e it t Al 3 0 0 0 Miles 0 1 2 3 No vertical exaggeration

  17. Future Failure • Evidence: • Tension Cracks • Mass is Extending to Free Face • Morphometry: • Height: 4,100 feet • Predicted Volume: 37 million yards3 (rock) • Predicted Length: 2-4 miles • Likely Trigger Mechanism: • Seismic Shaking

  18. Conclusions • Future Slide: • Actively Extending Rock Mass 4,000 feet Above Valley • It Will Happen • But When? • No evidence of catastrophic failure for ~12,000 yrs • … would you worry?

  19. “…as geologists, we have a good idea of how things work, but little idea of how quickly…” - Dick Berg, MBMG

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