AFM records small debris flows at Mount St. Helens after heavy September rain.

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On September 28, after about 10 hours of steady rain, an acoustic flow monitor (AFM) on the South Fork Toutle River recorded the passage of small debris flows.

The AFM showed significantly elevated signals for about three hours as water, mud and rock rolled downstream in small surges. During recent field work, crews visited the area to see the evidence -- remnant debris flow surfaces on channel margins, new deposits, mud lines and bent, scoured alders. The flow likely traveled for over a mile downstream of the AFM location. No damage to infrastructure was reported. Read more about sediment erosion, transportation and deposition at Hydrologic Monitoring at Mount St.Helens.