The spread of Phragmites australis between 1980 and 2002 was documented from seven series of aerial photographs and remote sensing images covering the Grandes Battures Tailhandier (Boucherville Islands, St. Lawrence River, Québec, Canada). Over the 23-y period, the colonized surface rose exponentially from 0.86 to 32.6 ha, corresponding to an 18% annual increase. This increase resulted mostly from vegetative growth, although the establishment of new colonies — most likely resulting from seed germination — allowed longer-range dispersion. Hydrological factors, especially the water level and duration of flooding over the growth season (July 1 to October 31) of the previous year, favoured the spread of colonies. Gains were highest the year following low water-level conditions and in a southerly direction, whereas they were reduced when plants grew at more than 1.5 m above mean water level or when they were flooded for more than 100 d during the previous growing season. The rate of surface colonization observed at Boucherville Islands was compared to that recorded at four other fluvial sites. Between Cornwall and Trois-Rivières, the noticeable increase in the number of colonized sites since 1980 suggests that low water levels in 1995, 1999, and 2001 favoured the establishment of colonies of P. australis along the shores of the St. Lawrence River.