LARGE WOODY DEBRIS JAMS, CHANNEL HYDRAULICS AND HABITAT FORMATION IN LARGE RIVERS

Field surveys document the accumulation of large woody debris (LWD) in to structurally distinctive jam types in the alluvial channel of the Q ueets River on the Olympic Peninsula of north west Washington. Calcula tions, field observations and historical evidence show that these jams can form stable structures controlling local channel hydraulics and p roviding refugia for riparian forest development over decades and poss ibly centuries. Distinctive spatial patterns of LWD, pools, bars and f orested islands form in association with particular jam types. The dep osition of 'key member' logs initiates the formation of stable bar ape x and meander jams that alter the local flow hydraulics and thereby th e spatial characteristics of scour and deposition leading to pool and bar formation. Historical evidence and the age structure of forest pat ches documents the temporal development of alluvial topography associa ted with these jam types. Bar apex jams, for example, are associated w ith a crescentic pool, an upstream arcuate bar and a downstream centra l bar that is the focus of forest patch development. Experimental and empirical studies in hydraulic engineering accurately predict channel scour associated with jams. Individual jams can be remarkably stable, providing long-term bank protection that creates local refugia for mat ure forest patches within a valley floor environment characterized by rapid channel migration and frequent disturbance. Processes controllin g the formation, structure and stability of naturally occurring LWD ja ms are fundamental to the dynamics of forested river ecosystems and pr ovide insights into the design of both habitat restoration structures and ecosystem-based watershed management.