A flood of unprecedented magnitude was recorded on the Grand River in southwestern Ontario on May 16–17, 1974. Peak discharge recorded at Galt has a return period of about 1 in 500 or a probability of 0.20 in any given year. Rare, high magnitude floods are of interest to the geomorphologist for the work they do on the terrain. Channel, floodplain, and valley side features produced by the 1974 flood were mapped and described on a reach of the Grand and its tributary, the Conestoga River. Erosional and depositional effects were minor except in the vicinity of man-made obstructions such as bridge piers where scour was evident. The minor geomorphic impact of the 1974 flood suggests that the Grand River valley is geomorphologically well-adjusted to handling large discharges. The findings coincide with observed effects of other high magnitude floods described in the literature.