Timber harvesting, mass erosion, and steepland forest geomorphology in the Pacific Northwest

Forest operations in mountainous regions of the Pacific Northwest have a major impact on soil-erosion processes. The mountains of the region are youthful, the area has undergone recent tectonic activity, and west of the crest of the Cascades annual precipitation may exceed 375 cm. Consequently, natural erosion rates are high. Heavy forest vegetation and the high infiltration capacity of many of the forest soils protect slopes from surface erosion. The combination of these factors results in mass erosion processes generally being the dominant mechanisms of sediment transport from hillslopes to stream channels. The principal mass erosion processes are slow, downslope movement involving subtle deformation of the soil mantle (creep) and discrete failures, including slow-moving, deep-seated slump-earthflow; rapid, shallow soil and organic debris ;movement form hillslopes (debris avalanches); and rapid debris movement along downstream channels (debris torrents) (Fig. 1A, B). In many areas, forest vegetation plays an important role in stabilizing slopes and reducing the movement rate and occurrence of these mass erosion processes. When timber is removed from marginally stable slopes, whether by natural processes such as wildfire and wind or by the activities of man, a temporary acceleration of erosion activity is likely.

Accelerated erosion due to forest land management activities may result in reduced productivity of forest soils over sizable portions of affected watersheds, damage to roads, bridges, and other structures, and adverse impacts on the stream environment downstream.