South Fork Skokomish Stream Corridor Rehabilitation

In 1952, a dam was proposed for the South Fork Skokomish River in the Olympic Mountains in Washington State. The floodplain area above the proposed dam was logged and large wood (LW) within the stream was removed, however the dam project was abandoned. In addition to this disturbance, conditions within the watershed have been degraded by past riparian timber harvest, removal of instream LW, and an extensive history of timber harvest and road construction.

A restoration strategy was established and implemented on a 1.6 km reach along the upper South Fork Skokomish River.  Restoration focused on restoring floodplain resiliency and resistance to flow as well as increasing stream channel and bank stability.  This will be followed by restoration of riparian vegetation.  The ultimate goal is to improve fish habitat and fisheries production.  Channel geometry equations, hydraulic modeling, stream flow patterns, and reference reaches were used to develop rehabilitation designs and implementation templates. Channel and floodplain stability was accomplished by constructing strategically placed log jams and LW structures on floodplains, tributary fans, and along the bankfull perimeter. Log jam construction and LW structure placement were designed to emulate stable natural LW accumulations, in order to reduce streambank erosion, increase floodplain roughness, promote establishment of riparian vegetation, and increase habitat for aquatic organisms and threatened fish species. The project used whole length second-growth trees without cable, bolts or anchors.  Approximately 2000 trees were pushed over and transported to the river via helicopter.  Excavators were used to construct 30 log jams.

Implementation took place during the summer of 2010.  In December 2010, the LW structures endured a ten-year flood event (15,000 cfs).  All structures were still present after the event and still functioning as designed.  Utilization of second-growth trees, 15–76 cm in diameter, for LW structures appears to be a readily available and cost effective option for large river system/reach scale restoration efforts.  Larger trees or logs are not necessarily better, particularly if the structure elements are well buried. Structure design and placement of key log pieces buried into streambanks, gravel bars, and floodplains for stability is a viable alternative to the use of traditional anchoring methods such as ballast, cables, or bolts.