Riparian forest disturbances by a mountain flood - the influence of floated wood

Large floods can have major impacts on riparian forests. Here we examine th e variability and spatial distribution of riparian forest responses along e ight third- to fifth-order streams following a large flood ( similar to 100 year recurrence interval) in the Cascade Mountain Range of Oregon, We cate gorized disturbance intensity (physical force) exerted on riparian trees du ring floods into three classes: (i) purely fluvial (high water flow only); (ii) fluvial supplemented by dispersed pieces of floating wood (uncongested wood transport); (iii) fluvial with movement of batches of wood (congested wood transport). These types of material transport and associated classes of disturbance intensity resulted in a gradient of biotic responses of dist urbance severity ranging from standing riparian trees inundated by high wat er, to trees toppled but still partially rooted, to complete removal of tre es. High within-stream and among stream responses were influenced by pre-fl ood stream and riparian conditions as well as flood dynamics, especially th e availability of individual pieces or congested batches of wood. Fluvial disturbance alone toppled fewer riparian trees than in reaches wher e floodwaters transported substantial amounts of wood. Debris flows deliver ed additional wood and sediment to parts of reaches of four of these study streams; riparian trees were removed and toppled for up to 1.5 km downstrea m of the debris flow tributary channel. Congested wood transport resulted i n higher frequency of toppled trees and greater deposition of new wood leve es along channel margins. The condition of the landscape at the time of a m ajor flood strongly influenced responses of riparian forests. Recent and hi storic land-use practices, as well as the time since the previous large flo od, influenced not only the structure and age of the riparian forests, but also the availability of agents of disturbance, such as large pieces of flo ating wood, that contribute to disturbance of riparian forests during flood s. Copyright (C) 2000 John Wiley & Sons, Ltd.