Riparian forest disturbances by a mountain ̄ood Ð the in ̄uence of ̄oated wood

Large ̄oods can have major impacts on riparian forests. Here we examine the variability and spatial distribution of riparian forest responses along eight thirdto ®fth-order streams following a large ̄ood ( 100 year recurrence interval) in the Cascade Mountain Range of Oregon. We categorized disturbance intensity (physical force) exerted on riparian trees during ̄oods into three classes: (i) purely ̄uvial (high water ̄ow only); (ii) ̄uvial supplemented by dispersed pieces of ̄oating wood (uncongested wood transport); (iii) ̄uvial 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 disturbance severity ranging from standing riparian trees inundated by high water, to trees toppled but still partially rooted, to complete removal of trees. High within-stream and amongstream responses were in ̄uenced by prēood stream and riparian conditions as well as ̄ood dynamics, especially the availability of individual pieces or congested batches of wood. Fluvial disturbance alone toppled fewer riparian trees than in reaches where ̄oodwaters transported substantial amounts of wood. Debris ̄ows delivered 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 downstream of the debris ̄ow tributary channel. Congested wood transport resulted in higher frequency of toppled trees and greater deposition of new wood levees along channel margins. The condition of the landscape at the time of a major ̄ood strongly in ̄uenced responses of riparian forests. Recent and historic land-use practices, as well as the time since the previous large ̄ood, in ̄uenced not only the structure and age of the riparian forests, but also the availability of agents of disturbance, such as large pieces of ̄oating wood, that contribute to disturbance of riparian forests during ̄oods. Copyright # 2000 John Wiley & Sons, Ltd.