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We monitored the recruitment of young-of-year zebra mussels (Dreissena polymorpha (Pallas, 1771)) each autumn at 13 locations within four river reaches along the length (100 km) of the Rideau River, starting in 1990, the year of its discovery in that river, until 2015. Sampling was conducted on bottom structures of locks or on seasonally exposed substrate during autumn drawdowns conducted by Rideau Canal staff. Twenty-six years of monitoring zebra mussels in that river revealed a distinct and persistent upstream–downstream pattern, with highest densities occurring in the two downstream reaches. A "lake effect" was observed at Long Reach, where veligers have ideal conditions for larval development. Highest densities occurred in the mid-1990s, comparable with those reported in the Laurentian Great Lakes during peak invasion (200 000 to 500 000+ mussels/m2). Although the most upstream reaches of the river had low recruitment rates and low densities initially (0.01 to 10 mussels/m2), annual recruitment progressively increased to higher values (10 to 1000+ mussels/m2) because more veligers from the Rideau Lakes and the river headwaters were produced and drifted into the system. This study is unique because it provides a thorough understanding of the 26 years of invasion history of the zebra mussel in a small river system. [ABSTRACT FROM AUTHOR]