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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5204

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Summary and Conclusions

Each winter, landslides mobilize from slopes throughout the Pacific Northwest. If winter storms produce above average rainfall, the threat for rapidly moving landslides increases substantially. As landslides enter streams, they greatly affect downstream water quality by causing large and sudden increases in turbidity. This study analyzes one such example where turbidity data exist for a known rapidly moving landslide, or debris flow.

On November 6, 2006, a rocky debris flow surged down the western flank of Mount Jefferson in the Oregon Cascades. Although coarse, boulder-sized material settled near the base of the mountain, high flows and flooding continued through parts of the drainage basins of Milk and Pamelia Creeks, sending a mud slurry into the North Santiam River. Investigation into the source of the event suggested that warmer than normal air temperatures and heavy precipitation caused a portion of a debris-covered, stagnant snowfield to collapse in the upper Milk Creek valley. As boulders, ice, and water flowed downslope, additional material was aggregated from previous debris flows, pyroclastic flows, and glacial moraine deposits. Mud run-up markings on trees indicated that the confined channel flood stage was greater than 2.4 meters deep, while flows outside the channel gradually thinned out atop the open valley floor. Fine-grained sediments entrained in floodwaters were transported through the North Santiam River into Detroit Lake. Future high flows will continue to erode the fine-grained material remaining on boulders, logs, and the valley floor.

Velocity calculations for the debris-flow event suggest that the multiple waves of debris and water that surged throughout the day reached 3.9 meters per second. Analyses of water-quality samples collected during the event indicated that an estimated 9,800 to 13,000 cubic meters of fine-grained sediment was transported downstream to the North Santiam River. A follow-up investigation of the debris flow indicated that debris and flood deposits covered about 0.45 square kilometers, depositing between 100,000 and 240,000 cubic meters of material. The debris flow altered the landscape by diverting streamflow from its original channel south to a new channel that was carved into the forest floor. This debris flow supplied enough suspended sediment to increase turbidity values in the North Santiam River above Detroit Lake to an estimated 35,000 and 55,000 Formazin Nephelometric Units. These turbidity values rank among the highest recorded (or estimated) in the North Santiam River basin since the inception of the USGS North Santiam River Basin Suspended-Sediment and Turbidity Study monitoring network in 1998.

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