MANAGING A RIVER WITH A HISTORY OF DAM DIVERSIONS AND LOGGING

Management decisions have included in-channel log removal, large scale watershed logging, and significant flow diversions from the Skokomish River, a large gravel bed channel in the Olympic Peninsula in Washington State. The South Fork Skokomish River and Vance Creek join to form the mainstem of the Skokomish River, which is then joined by the North Fork Skokomish River. The mainstem travels to Hood Canal, forming a delta where it enters the canal. Land use changes affecting the basin began with settlement in the mid-19th century. Logging has been active in the watershed since the 1860s, and peaked in the mid-20^th century. The construction of Cushman Dam in 1932 diverted much of the flow from the North Fork tributary for power generation. Over time the mainstem channel bed aggraded and the channel lost much of its formerly braided character. The management of forests and rivers has changed in recent decade. Logging in the upper watershed continues but has reduced dramatically since the 1980s. Increased and phased flow releases from the dam have been implemented since 2008.

The implications of the different management decisions over time were investigated for their impact on the channel planform, sediment transport rates, and flooding frequency. Hydraulics were modeled in 2D and coupled with changes to the channel bed predicted by 1D sediment transport modeling. USGS stream gage data were available at multiple locations along with bulk sediment transport samples and gravel bar surface grain size distributions. Repeat cross-sections enabled estimates of net erosion and aggradation over time and aided with model calibration. The incidence of flooding and bulk sediment transport were estimated for different release schedules from the reservoir on the North Fork and predicted into the future. Modeling and analyses focused on the history of alterations to the Skokomish system, future rates of channel aggradation, erosion, and flooding in the Skokomish River system, and the effect of increased North Fork tributary flows on the downstream Skokomish River Valley. There are indications that recent management decisions regarding forest and channel restoration have begun to influence the system positively.