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

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Mount Jefferson Debris Flow, November 6, 2006

Weather Conditions

Early November 2006 was a warmer than normal period accompanied by a tropical storm which brought heavy precipitation (table 1). Air temperatures near Mount Jefferson increased quickly as the storm front moved across the Pacific Northwest. During the first week of November, maximum daily air temperatures increased 9.4°C, and minimum daily air temperatures increased 21.1°C. The warm front brought heavy precipitation to the region, peaking with a total of 12.8 cm on November 7. These weather conditions were characteristic of a “Pineapple Express,” where central Pacific tropical air streams over the Northwestern United States (National Oceanic and Atmospheric Administration, 2005). This storm was responsible for flooding and numerous landslides all over the region, such as those on Mount Rainier (National Park Service, 2007) and Mount Hood (Pirot and others, 2007). Similar conditions occurred in February 1996 and led to the worst flooding in more than 30 years for much of the Pacific Northwest.

Because the warm temperatures and rainfall occurred early in the winter, the mountain had minimal snowpack. A small amount of snow accumulated during November 1 and 2, especially at higher altitudes; however, as the warm air temperatures caused the snow level to rise, rain fell directly onto higher altitude snowfields. This “rain-on-snow” situation augmented the heavy rainfall conditions from November 3 through 6 by adding additional water to the hydrologic system.

First Detection

During the storm, streamflow increased and became more turbid in most streams in the North Santiam River basin. Unlike other streams, however, the turbidity peaked on the North Santiam River at the North Santiam station on November 6, a full day before the peak rainfall and streamflow occurred elsewhere (table 2, fig. 4). The timing, magnitude, and location of the high turbidity event were immediately recognized as a possible debris flow or glacial outwash event, similar to major turbidity events from previous years (Sobieszczyk and others, 2007). Because turbidity was so high, it exceeded the maximum detection capability of the instream water-quality instrumentation; however, estimates from samples collected (fig. 5) during the event indicate that turbidity in the North Santiam River ranged from 35,000 to 55,000 FNU (Heather Bragg, U.S. Geological Survey, written commun., 2008).

Turbidity values for this event were based on a series of samples collected by an ISCO automatic pumping sampler. The first set of 1-L sample bottles (sample set A) were filled during the start of the event and replaced with a second set of bottles (sample set B) that captured the latter part of the high turbidity (fig. 5). Laboratory measurements from these samples supplied a conservative turbidity value for the event, whereas the peak turbidity was estimated between the sets based on trends in other water-quality parameters, such as pH and conductivity, collected from the North Santiam monitoring station.

Tracing the source of turbid water in the North Santiam River was facilitated by the fact that the tributary Pamelia Creek, about 10 mi upstream of the monitoring station, was visibly turbid (fig. 6). USGS and USFS personnel investigating Pamelia Creek discovered that a debris flow had mobilized down the Milk Creek drainage basin, supplying debris and sediment to both Milk and Pamelia Creeks and beyond. Streamflow at Milk and Pamelia Creeks remained extremely turbid for 12 to 18 hours during the debris flow event-period on November 6.

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