Abstract

Mass movements are dominant erosional processes in the western Cascades of Oregon. Recent mass movements have degraded water quality and disrupted roads and conduits in the Bull Run Watershed, which is the primary source of drinking water for the City of Portland. the morphology and distribution of mass movements is quantifiably related to lithological, structural, and stratigraphic variations in bedrock.

The base of the stratigraphic section in the Bull Run Watershed consists of 900 feet of middle Miocene, tholeiitic basalt flows of the Grande Ronde and Wanapum Basalt Formations of the Columbia River Basalt Group. These are overlain, with local unconformity, by 600 feet of epiclastic and pyroclastic deposits of the Rhododendron Formation. The Rhododendron Formation is overlain, in the west, by an eastward thinning wedge of fluvial siltstone, sandstone, and conglomerate of the Troutdale Formation. Greater that 2,000 feet of basaltic andesite and less abundant basalt and pyroclastics overlie all older units with local unconformity. In the east, Pliocene volcanic rocks are overlain, locally, by Quaternary basalt, andesite, and hornblende andesite flows. Surficial deposits include extensive talus in the east, thick landslide debris in the west, widespread colluvial forest soils, and scattered glacial outwash and moraine. Glaciation did not extend below 2,200 feet. Bedding of all geologic units is nearly horizontal. The Columbia River Basalt Group, and possibly the Rhododendron Formation, were deformed by gentle N60E-trending folds and a southeastward-dipping thrust fault. All bedrock units and older structures are cut by near vertical, northwest-trending shear faults and tectonic joints.

Chi-square goodness of fit tests indicate that mass movement distribution and morphology are strongly correlated to the distribution of geologic unity and to site location with respect to geologic contacts. The highest number of slump-earthflow failures per unit area occur on the Rhododendron Formation and older landslide debris, whereas the density of debris avalanches is greatest on flows of the Columbia River Basalt Group, which form the steep-walled canyons of the Bull Run River. Prehistoric, complex-massive failures have occurred where soft pyroclastics of the Rhododendron Formation are intercalated with more competent flow rock units. Smaller, recent mass movements have occurred along the contact between the pyroclastics and the overlying, older landslide debris in the North Fork and South Fork drainages, which are the most active drainages in the watershed in terms of recent mass movements. Contacts between pyroclastics and underlying or overlying flow rocks and between flow rocks and overlying Quaternary landslide debris are also associated with a high density of recent mass movements. The density of springs and seeps follows a similar distribution with respect to geologic contacts, and the effluence of groundwater on slopes is related to mass movement occurrence, particularly by debris avalanche.

Mass movement occurrence is favored by high clay content in soils and by abundance of smectite, and possibly hydrated halloysite, in the clay fraction. Soils of this type weather from pyroclastics of the Rhododendron Formation, Quaternary landslide debris, and interflow breccias and interbeds.

Jointing attitudes have only a minor effect of mass movement occurrence in the watershed. Failures are more likely where the attitude of joints dips steeply with the slope. Average slope gradients at mass movement sites vary between 58 and 85 percent with debris avalanches occurring on steeper slopes than slump-earthflows.

A stepwise multiple regression analysis indicates that the occurrence of mass movements is most strongly correlated with the inverse of distance between a site and a geologic contact, moderately correlated with slope gradient, and weakly correlated with soil texture.

Data generated by statistical tests are used to weight geologic factors in terms of mass movement hazard levels. Stratification of the weighted values produced a mass movement hazard map for the Bull Run Watershed.