Abstract:This study examines the effect of drawdown on the timing and magnitude of suspended solids and associated phosphorus export from a 12 ha reservoir located in an urbanized watershed in southern Ontario, Canada. Water level in Columbia Lake was lowered by 1Ð15 m over a 2-week period in November 2001. The total phosphorus (TP) concentrations ranged from 63 to 486 µg L 1 in Columbia Lake and 71 to 373 µg L 1 at its outflow. All samples exceeded the Provincial Water Quality Objective of 30 µg TP L 1 . Outflow concentrations of suspended solids and TP increased significantly with decreasing lake level and were attributed to the resuspension of cohesive bottom sediments that occurred at a critical threshold lake level (0Ð65 m below summer level). Suspended solids at the outflow consisted of flocculated cohesive materials with a median diameter (D 50 ) of c. 5 µm. Particulate organic carbon accounted for 8Ð5% of the suspended solids export by mass. A total mass of 18Ð5 t of suspended solids and 62Ð6 kg TP was exported from Columbia Lake, which represents a significant pulse of sediment-associated P to downstream environments each autumn during drawdown. The downstream impacts of this release can be minimized if the water level in Columbia Lake is lowered no more than 0Ð5 m below summer levels.