The Fraser River in British Columbia, Canada, is the longest non-dammed river on the west coast of North America and supplies 20 × 106 t/yr of sediment to the Pacific Ocean. Abundant geomorphological evidence indicates that the Fraser River reversed its course to southward flow in the recent geological past. Investigation of two volcanic dams at Dog Creek demonstrates northward flow of the Fraser until at least 1.06 Ma, before reversal and erosion of the 270-km-long Fraser Canyon. We propose that the submarine Nitinat Fan off the coast of British Columbia records the reversal and sudden input of coarse continental-derived sediment ca. 0.76 Ma. This study confirms reversal of the Fraser River and places a firm constraint on the maximum age of that reversal. Reversal likely followed stream capture in response to enhanced glaciofluvial erosion and uplift of the Coast Mountains.