The black cottonwood, Populus trichocarpa, provides the foundtaion for the riparian woodlands throughout southern British Columbia (B.C.) and western Montana (MT). To study the interaction of riparian dynamics and cottonwood ecology, the present study investigated the influence of the extreme 1995 Elk River flood on the riparian zone and cottonwood ecology, and the effects of flood flow attenuation by the Libby Dam on riparian processess and cottonwoodland ecology of the Kootenai(y) River. Four river reaches were studied: the free-flowing Elk River near Fernie, B.C., the free-flowing Upper Kootenay River, B.C., upstream from the Koocanusa Reservoir, the free-flowing Fisher River, MT, near the Kootenai junction, and the flow-attenuated Lower Kootenai River near Libby, MT. Air photos from 1930, 1962, 1992 and/or 1994 revealed substantial channel change and the development of barren point bars that served as recruitment sites for willos and cottonwoods along the free-flowing reaches. Conversely, the Lower Kootenai had a relatively static channel after damming. In total, thirty-five transects were studied in 1996 and 1997 at 3 sites along each river reach to assess elevation profiles, substrate composition, scour and deposition, vegatation patterns, and aspects of cottonwood reproduction. Abundant cottonwood recruitment occurred in 1996 and even more so in 1997, producing mean densities of 153, 536, and 142 seedlings/m2 along the Elk, Upper Kootenay, and Fisher river transects, respectively. In marked contrast, no seedlings were successful along the Lower Kootenai River, downstream from the Libby Dam. The free-flowing river reaches experienced extensive sediment deposition in the riparian zone after the 1997 high water, whereas the Lower Kootenai experienced little change in stream bank configuration. The Elk River study revealed that the 1995 flood caused considerate geomorphic change and the resultant unconsolidated deposits were easily scoured and transported during the subsequent two years providing abundant sites for new cottonwoods. The Kootenay River study revealed limited meandering and deposition along the flow-attenuated Lower Kootenai River compared to the hydrologically, geomorphologically, and ecologically dynamic, free-flowing upstream reach. Along the Lower Kootenai River, there was a deficiency in black cottonwood population age structure due to limited recruitment. Flood-intolerant, upland plants have encroached to the river's edge, further eliminating cottonwood recruitment opportunities along the Lower Kootenai River. The vegetation encroachment, the static channel configuration, the minimal scour and sediment deposition and the lack of the essential stream stage pattern, combine to underlie the lack of seedling recruitment and the consequent deficiency in cottonwood population structure along the Lower Kootenai River. The studies demonstrate that black cottonwoods require a dynamic hydrologic and geomorphic system with periodic flood events for continued replenishment. The observed loss of cottonwood recruitment along the Lower Kootenai River is thus the consequence of the flood flow attenuation due to the operation of the Libby Dam. The restoration of the Lower Kootenai cottonwoods will probably rely on a partial recovery of more natural and more dynamic instream flow patterns that include occasional high flows in late spring followed by gradual stage recession. Such flows would exclude upland vegetation, recover more dynamic geomorphic processes and provide the stream stage patterns that are directly essential for cottonwood seedling recruitment. / 1 v. (various pagings) : ill. (some col.), maps ; 28 cm.