. Mass wasting in coastal Alaska. Mass-wasting Alaska. Downslope, a debris avalanche fre- quently becomes a debris flow be- cause of substantial increases in water content. OCCURRENCE AND DISTRIBUTION Inspection of aerial photos of the Tongass National Forest, cover- ing approximately 99 percent of the southeast Alaska land area, reveals evidence of widespread active mass 3 / wasting in timbered areas.—7 More than 3,800 large-scale debris ava- lanches and flows have been counted, — Landslide occurrence in coastal Alaska. Study No. 1604-12; data on file, Pacific Northwest Forest and Range Exper
. Mass wasting in coastal Alaska. Mass-wasting Alaska. Downslope, a debris avalanche fre- quently becomes a debris flow be- cause of substantial increases in water content. OCCURRENCE AND DISTRIBUTION Inspection of aerial photos of the Tongass National Forest, cover- ing approximately 99 percent of the southeast Alaska land area, reveals evidence of widespread active mass 3 / wasting in timbered areas.—7 More than 3,800 large-scale debris ava- lanches and flows have been counted, — Landslide occurrence in coastal Alaska. Study No. 1604-12; data on file, Pacific Northwest Forest and Range Experiment Station, Institute of Northern Forestry, Juneau. most having occurred within the last 150 years. Evidence of older sliding, predating the present old-growth forest, is masked from air photo identification but has been observed in the field in the form of buried soils and overturned profiles. The most recent debris avalanches and debris flows are clearly identified by bare linear strips on valley sideslopes and in deep V-notch channels where soil and vegetation have been removed (fig. 1). Increasingly older avalanche and flow scars are identified successively by strips of pioneering species such as willow ( Salix spp. ) and alder (Alnus spp. ) and finally by even-age stands of Sitka spruce (Picea sitchensis (Bong. ) Carr.) which are younge r than the sur- rounding timber (figs. 2 and 3). Such B. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Swanston, Douglas N. Juneau, Alaska : Pacific Northwest Forest and Range Experiment Station, U. S. Dept. of Agriculture, Institute of Northern Forestry
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