LARGE WOODY DEBRIS, PHYSICAL PROCESS, AND RIPARIAN FOREST DEVELOPMENTIN MONTANE RIVER NETWORKS OF THE PACIFIC-NORTHWEST

We present a conceptual biogeomorphic model of riparian forest develop ment in montane river networks. The role of physical process in drivin g the structure, composition, and spatial distribution of riparian for ests is examined. We classify the drainage network into disturbance pr ocess-based segments including: (1) debris-flow and avalanche channels , (2) fluvial and debris-flow channels, and (3) fluvial channels. Ripa rian forests are shown to be significant in the development of channel morphology through the stabilization of active floodplains and as sou rces of large woody debris (OLWD). LWD is operationally defined as woo d > 0.1 m diameter and > 1 m length. LWD plays a key role in the devel opment of montane riparian forests. LWD deposited in the active channe l and floodplain provides sites for vegetation colonization, forest is land growth and coalescence, and forest floodplain development. Ripari an forest patterns parallel the distribution of hillslope and fluvial processes through the network. Riparian forest structure, composition, and spatial distribution through the network are driven by the major disturbance processes including: (1) avalanches, (2) debris-flows, and (3) flooding. Riparian forest patterns also reflect the action of LWD in the organization and development of forested floodplains in gravel bedded montane river networks. The focus of our examples are montane river networks of the Pacific Northwest, USA.