FIELD INVESTIGATIONS OF LOGJAM DISTRIBUTION AND DYNAMICS BEFORE AND DURING THE ELWHA RIVER RESTORATION PROJECT DAM REMOVALS

The interactions between large wood debris, fluid flow and sediment transport in rivers are first order controls on river processes, affecting channel roughness, streambed morphology and sediment transport. Large wood debris increases the hydraulic and topographic complexity in rivers, leading to a greater diversity of aquatic habitats and an increase in the number of large pools that are important fish habitat for feeding, hiding and spawning. We investigate logjam distribution and dynamics on the Elwha River before and during the Elwha River Restoration project dam removals, as affected by discharge, sediment load, increased upstream sources of wood, changes in local wood recruitment and underlying geomorphic conditions. Logjam size, location and geomorphic setting are measured through field surveys and air photos. Prior to dam removal, logjam dynamics were dominated by local recruitment during bank erosion, channel migration and avulsions. Several large logjams formed in side channels during floods, and have been gradually revegetating, producing new floodplain forest. Removal of the Elwha Dam and Glines Canyon Dam have resulted in an increased downstream flux of sediment and wood that was previously stored in the reservoirs behind the dams. During dam removal, we see an increase in logjam area in the channel main-stem, rapid development of logjams in the former Lake Aldwell reservoir, and very little logjam development in the former Lake Mills reservoir. In the river channels between Aldwell and Mills, logjam area increased with increased upstream wood flux and more local recruitment as the higher sediment load increased lateral channel mobility. We hypothesize that logjam development in the two former reservoirs followed very different trajectories due to the Aldwell reservoir storing less sediment and therefore eroding more rapidly, there being a greater upstream flux of wood into Aldwell, and that Mills has pre-dam valley floor tree stumps of less than 1 m diameter while Aldwell had a significant number of pre-dam valley floor tree stumps 1 -3 m in diameter that became anchor points for logjam formation. In the next few years we expect more logjams to develop in Mills, but at a slower rate than logjams developed in Aldwell, since upstream wood flux into Mills will continue to be lower than upstream wood flux into Aldwell.