Small-format aerial photography of Squamish River just prior to and immediately following the October 1984 flood of record forms the basis of a quantitative evaluation of floodplain erosion and construction during this extreme event. Channel changes in 1984 are compared with those determined from sequential aerial photography at various other times since 1947 and associated with less extreme flooding.Depending on the type of river planform, the degree of channel change during the 1984 flood varied from a relatively minor response to a major reorganization of the channel. Despite its large size, the 1984 flood accomplished little more floodplain modification in the meandering and transitional semibraided reaches than had previous smaller floods of similar duration. In general, greater erosion was accomplished here by relatively small but longer duration flood events. In contrast, in the braided reach the 1984 flood caused floodplain erosion and major reorganization of the channel to an extent previously unrecorded, apparently here exceeding a threshold for channel stability. For all reaches, variation in floodplain erosion among sites was greater than at-a-site variation in erosion related to flood history.