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Scientific Investigations Report 2007–5178

Scientific Investigations Report 2007–5178

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Sedimentation Processes

Precipitation-Driven Landsliding

Analysis of the major turbidity events in the North Santiam River basin revealed a few general trends. Most major turbidity events were initiated by rainfall. The largest turbidity events also often incorporated multiple sediment sources; however, in each case, at least one discrete contributing source area existed. In most cases, landslides (as earthflows or debris flows) were the most common sediment sources. In general, landslides developed where rainfall saturated heavily reworked and weak subsurface rocks or sediments, such that high pore-water pressures exceeded the residual strength of the saturated material (Dikau and others, 1996). Other factors that also may have caused landsliding include geologic setting, slope characteristics, hydrologic properties of the soils, subsurface flow, characteristics of the surface vegetation, and land-management practices such as timber harvesting, road construction, and residential development (Sidle and others, 1986).

Snowmelt-Driven Glacial Outwash

The catalyst for two of the major turbidity events was increased snowmelt. During warm conditions (as in October 2000 and 2003), High Cascade snowpack melted, either releasing stored material or eroding away loose, unconsolidated soils. In addition, sometimes as part of the autumn melt, water and sediment may have been released from receding glaciers. Although not as severe as glacial outburst floods that can occur in the Cascades (Walder and Driedger, 1994; O’Connor and others, 2001), the increased runoff would erode existing High Cascade landslides. The seasonality of the glacial events separated them from the other sediment sources in the North Santiam River basin that mobilized during rainy winters. Turbidity signatures from the glacial or snowmelt-driven events were unique because they were associated with minor or no increase in discharge. This likely was a result of the low streamflow from Milk Creek as compared to that of the upper North Santiam River. Determining the exact contributing area in the Mount Jefferson Wilderness Area was problematic because the terrain was remote and inaccessible. Rapidly changing environmental conditions also limited remote sensing opportunities.

Anthropogenic Influences

Besides natural characteristics, anthropogenic factors also influenced sediment production in the basin. For example, road construction increased surface runoff, and in at least one case, promoted landsliding (see section, “December 17-19, 2001-Blowout Creek”). In general, road construction changes slope stability and hydrologic drainage by adding weight to the slope with embankment fill, increasing steepness of the slope on the cut and fill surfaces, reducing the support for a cutslope, and rerouting and concentrating surface water (Sidle and others, 1986; Montgomery, 1994; U.S. Forest Service, 2001). As with other types of mass wasting, sedimentation resulting from road-related landsliding most often occurred during periods of intense or prolonged rainfall (Wemple and others, 2001).

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