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

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Introduction

Every year, winter storms cause tens to hundreds of debris flows in the Pacific Northwest (Beaulieu and Olmstead, 1999; Hofmeister, 2000; National Park Service, 2007; Pirot and others, 2007; Burns and others, 2008; Scott Burns, Portland State University, oral commun., 2008). This number increases significantly during years of heavy rainfall and flooding, when thousands of debris flows may be observed. For example, during the floods of 1996 and 1997, more than 9,500 debris flows, or rapidly moving landslides, were mapped in Oregon alone (Hofmeister, 2000). Landslide activity in Oregon is so common that the Oregon Department of Forestry administers a debris flow warning system. This system issues warnings through the National Weather Service whenever conditions are deemed unsafe along or beneath certain steep hillsides, such as during heavy rainfall or after rapid snowmelt (Oregon Department of Geology and Mineral Industries, 2008). Although debris flows are dangerous, most are small, travel short distances, and occur in remote regions; however, sometimes they impact populated areas and damage infrastructure.

Although debris flows are common in Oregon and the Pacific Northwest, there is limited research investigating the relation between debris flows and downstream water-quality characteristics, such as turbidity. Where such research does exist, the discussion about turbidity usually focuses only on whether the debris flow influenced-streamflow is hyperconcentrated, not the effect the turbid water has downstream. Because of the adverse effects that turbidity can have on drinking-water operations, further investigation into the relation between landslides and turbidity is needed, especially in environments where debris flows occur proximate to reservoirs and other drinking-water sources. One such environment where landslide hazards coincide with hydrologic concerns is in the North Santiam River basin. After the flooding and landslide damage of 1996 and 1997, the U.S. Geological Survey (USGS) in cooperation with the City of Salem, established a continuous near real-time water-quality monitoring network in the basin (Uhrich and Bragg, 2003). This network allows the USGS to monitor temperature, specific conductance, pH, turbidity, and streamflow at 10 water-quality monitoring stations (U.S. Geological Survey, 2008). Using this continuous, near real-time network, the USGS and others have detected several high turbidity events and traced them to various sources, including debris flows, earthflows, and road failures (Sobieszczyk and others, 2007).

The largest recorded high-turbidity event for the North Santiam River, through December 2007, occurred November 6, 2006 (fig. 1), at the North Santiam water-quality monitoring station (14178000; fig. 2). After tracing the turbid water to its source tributary, Pamelia Creek, the USGS and the U.S. Forest Service (USFS) concluded that the turbid water resulted from a debris flow that mobilized off the western slope of Mount Jefferson. Debris and sediment from this event were carried down Milk Creek into Pamelia Creek and finally to the North Santiam River and Detroit Lake. In addition to the suspended sediment transported downstream, a large volume of material was reworked and deposited in the drainage basins of Milk and Pamelia Creeks. Temporary storage of this recently deposited, highly erodible material likely will increase turbidity during future high streamflow events.

Purpose and Scope

This report describes the geomorphology of the Mount Jefferson debris flow that occurred on November 6, 2006, and its effect on the downstream turbidity and suspended-sediment load in the North Santiam River. This event represents one of the few examples where water-quality data, such as turbidity, directly relates to a known sediment source, such as a debris flow (Sobieszczyk and others, 2007). Data and measurements for this event were quantified by a combination of field survey data, precipitation data, water-quality data, remote sensing data, and aerial photography. Turbidity values referenced in this report were recorded as part of the USGS North Santiam River Basin Suspended-Sediment and Turbidity Study monitoring network (http://or.water.usgs.gov/santiam/)).

Description of Study Area

Mount Jefferson with an altitude of 3,200 m is the second tallest Western Cascade volcano in Oregon. The mountain and its accompanying wilderness area are about 100 km east of Salem, Oregon. Similar to most volcanic environments, there are multiple hazards associated with the mountain that potentially endanger people and property. Beyond the remote hazard of a large explosive volcanic eruption (last occurring more than 35,000 years ago; Walder and others, 1999) or basaltic lava flows (last occurring more than 7,600 years ago; Walder and others, 1999), other more likely hazards include landslides and snow avalanches. Although stable bedrock limits deep-seated landslides, the steep slopes and unconsolidated glacial moraine and pyroclastic flow deposits are susceptible to rock falls and debris flows. For example, at least two debris flows have mobilized from the western slopes of Mount Jefferson down the Milk and Pamelia Creek drainage basins prior to 2006 (Sobieszczyk and others, 2007). There also are accounts of debris flow activity in the Milk Creek drainage basin in the 1990s (David Halemeier, U.S. Forest Service, oral commun., 2008).

Milk and Pamelia Creek drainage basins are located on the western flank of Mount Jefferson, within the Mount Jefferson Wilderness Area (fig. 3). Pamelia Creek drains 63 km², conveying a large portion of the runoff from the western slope of Mount Jefferson. Milk Creek is within the Pamelia Creek drainage basin and drains 6.5 km², with flows originating from remnants of the Milk Creek Glacier, high-altitude valley snowfields, and ground water. Although the upper drainage basins are above treeline, the western lower altitude portions are densely vegetated with western white pine, western hemlock, Douglas-fir, silver fir, and rhododendrons in the understory (U.S. Forest Service, 2007; David Halemeier, U.S. Forest Service, oral commun., 2008). Vehicle access within the drainage basins is limited, but a large part of the Mount Jefferson Wilderness Area is accessible by trail, such as the Pamelia Lake and Pacific Crest Trails.

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