EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 15, 457-466 (1990) THE INFLUENCE OF DEBRIS FLOWS ON CHANNELS AND VALLEY FLOORS IN THE OREGON COAST RANGE, U.S.A. LEE BENDA Department of Geological Sciences (AJ-20 ), University of Washington, Seattle, Washington 98195, U.S.A. Received 24 August 1989 Revised 2 January 1990 ABSTRACT Debris flows are one of the most important processes which influence the morphology of channels and valley floors in the Oregon Coast Range. Debris flows that initiate in bedrock hollows at heads of first-order basins erode the long- accumulated sediment and organic debris from the floors of headwater, first- and second-order channels. This material is deposited on valley floors in the form of fans, levees, and terraces. In channels, deposits of debris flows control the distribution of boulders. The stochastic nature of sediment supply to alluvial channels by debris flows promotes cycling between channel aggradation which results in a gravel-bed morphology, and channel degradation which results in a mixed bedrock- and boulder-bed morphology. Temporal and spatial variability of channel-bed morphology is expected in other landscapes where debris flows are an important process. KEY WORDS Debris flows Mountain channels Channel aggradation INTRODUCTION Debris flows in the west coast of North America form as landslide debris liquifies and moves through steep, confined, headwater channels for distances up to several kilometres at speeds up to 10-15 m s t . The flows are extremely erosive and incorporate additional sediment from channels, as well as or ganic debris and water, and they deposit this material in lower-gradient channels and valley bottoms. The steep, dissected mountains of the west coast of North America favour the formation of debris flows (Eisbacher and Clague, 1984: Swanson et al.. 1987). Debris flows are an important sediment transport link between hillslopes and alluvial channels and thus are an important factor in basin sediment bud gets (Dietrich and Dunne, 1978: Swanson et al., 1982: Benda and Dunne, 1987). In addition, debris flows influence the spatial and temporal distribution of sediments in alluvial channels either because they deposit sediment in alluvial channels, or because the deposits provide a source for accelerated transport of sediment further downstream. Despite the importance of debris flows in the transport of sediment from headwater channels, little is known of their role in shapin g the morphology of channels and valley floors in the northwest region of North America. In this paper I present a field study of the erosional and depositional characteristics of debris flows and their influence on alluvial channels and valley floors. STUDY BASIN Forty-four debris flows were examined in Knowles Creek, a 52 km 2, fifth-order basin underlain by marine sandstones of the Tyee and Flournoy formations in the central Coast Range of Oregon, U.S.A. (Baldwin, 0197-9337/90/050457-10S05.00 (0 1990 by John Wiley Sons, Ltd. 458 L. BENDA Eugene • Knowles Creek Coos Bay V Figure 1. Study area-in the Oregon Coast Range, U.S.A. 1964) (Figure 1). The Tyee and Flournoy formations are massive, rhythmically-bedded sandstones with interbeds of siltstones. Hillslopes have gradients of 35° to 45° and are sculpted into bedrock hollows (Dietrich and Dunne, 1978) also referred to as zero-order basins (Tsukamoto et al., 1982). Colluvium on hillslopes ranges in depth from 0. 3 m to 0. 5 m, and in hollows from 0 .4 m to 3 . 5 m. Bedrock hollows lead into steep, partially-vegetated headwater channels with bouldery and cobbly beds. Benda and Dunne (1987) examined the texture of sediment in these first- and second-order channels, and concluded that, apart from a sorted surface pavement, the deposits consisted primarily of colluvium supplied from the adjacent hillslopes. Colluvium is transported to stream channels by a variety of processes which include rheological soil creep, treethrow, animal burrowing, and small streamside landslides. Depths of colluvium which range from one to three metres in headwater channels constitute a source of sediment to augment the volume of scouring debris flows. Annual precipitation of 1600 mm, falling almost entirely as rain during winter, supports dense stands of Douglas Fir (Pseudostuga menziesii) and western hemlock (Tsuga lieterophylla). Road construction and clearcut logging began intensively during the 1950s and continues to the present. Recent large rainstorms in conjunction with land-use activities have resulted in at least 44 debris flows in the Knowles Creek drainage over the last 30 years. CHARACTERISTICS OF DEBRIS FLOWS Bedrock hollows: loci for initiation of debris flows In the highly-dissected basins of Knowles Creek, debris flows were initiated by landslides which occurred in colluvium-filled bedrock hollows in 78 per cent of the 36 cases where initiation sites could be identified. The remaining 22 per cent of failures occurred along relatively planar hillslopes. Initiation of debris flows was not observed to occur in headwater channels. The spatial density of hollows in the study basin averages 100,/km 2 .The number of hollows in a first-order basin averages seven. Two to five bedrock hollows (average four) located at heads of first-order channels, and entering stream channels at angles less than 45°consistently initiated debris flows; these have been referred to as trigger hollows (Benda and Dunne 1987). The other three hollows in a typical first-order basin, and those in second-order basins (7) enter channels approximately at right angles and the sediment from them deposit in first- and second-order channels. DEBRIS FLOWS IN OREGON 459 Debris flows travel through first-, second-, and third-order channels. Ninety-three per cent (40) of all debris flows reached at least the mouths of first-order basins, an average distance of 250 m. Eighty-two per cent (36) continued to travel through second-order channels, increasing their travel distance to an average of 550 m. Forty-one per cent (18) entered and traversed at least a portion of a third-order channel, increasing their average distance to 1050 m. Erosion of headwater channels by debris flows A comparison of sediment sizes from several locations in the basin indicates that deposits in first- and second-order channels are dominated by colluvium (Figure 2). The sediment size distributions shown in Figure 2 are the averages of combinations of surface counts (Wolman, 1954), and bulk sieve analyses for hollows (n = 4), first- and second-order channels (n = 4) in two second-order basins, debris flow deposits (n = 3) at second-order tributary junctions, and surface and freeze-core sampling (Everest and Meehan, 1981) in fourth-order streams (n = 4). The freeze-core method samples the armour layer and 0 . 3 m into the underlying sediment. Similar proportions of sediment finer than coarse sand ( < 1 .0 mm) in first-order channels (12 per cent) and second-order channels (14 per cent), and bedrock hollows (9 per cent) suggest little selective transport or abrasion by fluvial processes in headwater channels. The texture, shape, sorting, and cohesion of sediments in headwater channels which indicates their colluvial origin is discussed in detail by Benda and Dunne (1987) and by Benda (1988). In Knowles Creek basin debris flows completely scour the sediments from the floors of first- and second- order channels exposing the channel bedrock in almost all cases. To estimate the volume of sediment stored in first- and second-order channels and thus the amount eroded by debris flows, I measured the cross- sectional areas of surviving deposits of colluvium in channels, and multiplied the areas by the lengths of channels. These measurements were made in four first-order and two second-order channels. Based on these measurements, debris flows erode 5 to 10 m 3 of sediment per metre length of channel (average 8 m 3 m 40 % Hollow 20 401 First- order 20 40 % Second- 20 order Debris Flow 20. 401 Fourth- 20 order 0.06 0.25 10 22 64 256 Particle Sizes (mm) Figure 2. Sizes of sediment from various locations in Knowles Creek basin 460 L. BENDA Erosion of channel sediments increased the volume of debris flows from an average of 450 m 3 (range 35 m 3-1200 m 3 ) at the initial failure to 2400 m 3 (range 1700 m3-3000 m 3 ) at the mouths of first-order channels. Debris flows which continued to erode throughout second-order channels increased their average to 4000 m 3 (range 3400 m 3 -6000 m 3 ). Erosion by debris flow continued into third-order channels, but usually less than a distance of 100 m. There are several reasons why sediment delivered to first- and second-order channels resists fluvial erosion and accumulates until eroded by a debris flow. Channels scoured by debris flows commonly refill by landslides from hollows and hillslopes. Instantaneous delivery of thick wedges of sediments to narrow channels reduces the opportunity for streamflow to erode and transport sediments. In addition. headwater channels have large boulders and both live and dead large woody material on surfaces of deposits which increases roughness and reduces gradients thereby reducing sediment transport. Fluvial sediment transport in headwater channels is minimal also because water velocities in streams are insufficient to transport the coarse armour layer that protects the underlying sediment from stream erosion. Deposition of debris flows Deposition of debris flows occurs in third- through fifth-order channels and valley floors typically at mouths of first- and second-order basins. Deposits consist of less than 1000 m 3 of organic debris followed by a 50 to 150 m-long accumulation of unsorted sediments ranging in width from 10-20 m and in thickness from one to two metres. The deposits. which completely fill alluvial channels, are composed of gravels, cobbles.