Riparian zones are especially dynamic due to occasional river floods that provide powerful physical disturbances. Northern Hemisphere riparian woodlands are often dominated by cottonwoods, and studies in semi-arid ecoregions have indicated associations between flood events and cottonwood colonization. Here, we investigated the association between a major flood and cottonwood recruitment along a mountain river in a humid, forested ecoregion. We investigated riparian processes and black cottonwoods (Populus trichocarpa) along the free-flowing Elk River in the Rocky Mountains of British Columbia between 1993 and 2005, a period with a 1-in-100 year flood in June 1995. We analyzed hydrology, floodplain conditions, and aerial photographs and monitored vegetation establishment and survival. We excavated cottonwood saplings to determine origin and considered spatial patterns of cottonwood colonization. Results revealed that the flood induced channel migration and abrupt avulsion that created extensive barren nursery sites. This enabled prolific cottonwood establishment, but few seedlings from the flood year survived. Survival increased in subsequent years, but mortality remained extensive due to sediment burial, scour by high flows and ice, and drought. The patterns of successful colonization revealed seedling safe sites, survivable locations with appropriate elevation relative to water (0.6 to 2.8 m above base stage) and geomorphic context (<5 cm scour and <40 cm deposition). By 2002, cottonwood saplings covered islands and bars created by the flood, and excavation revealed seedling origin, not from clonal fragments. Clonal root suckers were stimulated by flood scarification, expanding previously established groves. The study confirmed that a major flood enabled extensive cottonwood colonization, but the underlying processes were somewhat different than along adjacent rivers in semi-arid ecoregions. Along the Elk River, sediment erosion and deposition were predominant, producing patches of even-aged cottonwoods, in contrast to the arcuate banding along rivers in drier regions with finer sediments and progressive channel migration. Thus, water was less limiting in the humid ecoregion and geomorphic disturbance was the more vital component of the flood event.