Upper Rideau Lake is a major lake in the Rideau Canal system; a waterway constructed in 1832 to link Kingston with Ottawa. The lake is currently borderline eutrophic, and local residents are concerned about its water quality. Long-term water chemistry data, however, are not available, and so we used diatom-based paleolimnological techniques to reconstruct the lake's development over the last two centuries. Our data indicate that Upper Rideau Lake was moderately productive even before European settlers began to affect this part of Ontario. The construction of the Rideau Canal, the first major anthropogenic disturbance in this system, greatly affected water quality. During the construction of the canal, the water level in Upper Rideau Lake was raised 1.5 m, resulting in increased nutrient and sediment loading, and an enlarged littoral zone. Deforestation associated with timber harvesting and agricultural activities maintained higher nutrient levels between ca. 1830 and the 1940s. Nutrient levels increased again over the last 40 years, with increased cultural activities in the drainage basin. Notwithstanding these more recent disturbances, the construction of the Rideau Canal in the early part of the 19th century had the largest impact on this lake.