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. . Tamarisk Invasion . An additional historic change in fluvial geomorphology has been the invasion of tamarisk (Tamarix ramosissima). Since its introduction at the turn of the century, tamarisk has invaded the Colorado River drainage at a rate of 20 km per year (Graf 1978). Dense tamarisk stands on flood plains may: decrease flood flow velocities; increase flow depth and roughness characteristics; increase channel storage; change width/depth characteristics to a narrow, deep configuration; and significantly attenuate flood peaks larger than can be contained in the channel (Singh 1987). Although the consequences have not been unexplored, altered channel morphology may influence backwater formation and the reproductive success of vegetation and both native and introduced fish species. Tributary Flooding and Sedimentation Debris flows and flash floods are responsible for transporting large loads of sediments from tributaries into the mainstream. Debris flows are flood slurries containing relatively little water but transporting large quantities of clay to boulder- sized particles. In contrast, flash floods contain more water and are less able to transport large boulders. Webb et al. (1987a) found that debris flows in the Grand Canyon were initiated in slope failure events, particularly in the soft Pennsylvanian Esplanade and Permian Hermit Shale during heavy precipitation when cliff faces were saturated. For example, Crystal Creek (rk 158) sustained a major debris flow on December 1966, following a rain- on-snow storm on the North Rim. Boulders driven from this event turned a minor rapid into an awesome cataract overnight. Webb et al. (1987a) estimated that one debris flow reached the Colorado River every 20 to 50 years, but weather patterns may be responsible for simultaneous events. Debris flow frequency was unpredictable in the Grand Canyon because exceptionally heavy summer monsoons and canyon profile may generate create2debris flow events even in small canyons. For example, the 5.2 km basin of Royal Arch Creek sustained a debris-flow that transported bus- sized boulders in 1985 (Webb et al. 1987a). In contrast with debris flows, flash floods are smaller- scale, more regular events which are generated by summer monsoons, and deliver considerable quantities of fine sediments to the Colorado River (Schmidt and Graf 1987). From an ecological perspective, debris flows and flash floods constitute significant disturbance events, scouring vegetation out of tributary channels and opening new habitat patches for plant colonization on a periodic basis. Depending on their severity, flash floods may not result in significant mortality of established plants and are of . . . . . . . 32 .