CUMULATIVE EFFECTS OF LOGGING HISTORY AND SLASH ACCUMULATION ON CHANNEL STABILITY IN SENSITIVE SOILS ON THE OLYMPIC PENINSULA, WA

As a forestland manager, the Washington State Dept. of Natural Resources (WADNR) is responsible for managing over 8,000 km² of forested land in Washington State. One management goal is to avoid increasing the occurrence of mass-wasting events that commonly impact watersheds and associated infrastructure. When necessary, geologists are consulted to assess current slope stability conditions as well as predict future slope stability conditions with respect to proposed management activities. Most of this assessment has been done on an activity and site-specific basis, without much analysis on a larger scale. The WADNR is currently applying landscape level planning as an approach to forestland management. Given the scale of this planning, aerial photo review and interpretation are relied upon for gathering slope stability information.

Air photos were reviewed for 215 km² of WADNR managed land on the western Olympic Peninsula. This landscape level project was conducted to inventory and delineate the potentially unstable landforms existing on the landscape. The photo review, done during the summer and fall of 2002 using air photos flown in 1997, identified 2,811 potentially unstable features of which 1.3% were field verified.

Field review of potentially unstable landforms identified on aerial photos for this project often led to a reevaluation of existing slope stability conditions originally based on aerial photo interpretation. The office review identified features with a lower gradient, and lacking the density of trees and vegetation of the surrounding area. These features occurred in a loose dendritic pattern. In the field, these areas were observed to have large quantities of cedar slash remaining from previous logging activities. The cedar is thought to have been transported to these areas during periods of heavy precipitation when surface water was most likely abundant. Deeply incised channels are common, some of which are slumping. In some cases, the old slash is redirecting stream flow into channel banks and causing increased slumping and incision. Management decisions in these areas should take into account the effects of increased peak flows on channel incision and channel slumping.