Descending 4.7 m over a length of 658 m, Sly’s Rapids were named after William Sly, an early settler who arrived in the Smith Falls area in 1798. By 1824 he had built a substantial barn and house on the north side of the river near to where Rideau Canal locks 26 and 27 were to be constructed.
The two locks, with a combined lift of 16 feet, 6 inches (5 m), were built with bedrock floors. However, blasting in the upper lock fractured the bedrock to such an extent that By had a wood plank floor anchored to the bedrock in that lock, in order to make it watertight. This is the only lock on the Rideau to have had a wood floor placed directly on bedrock.
What appears to be a retaining wall below the road that crosses the locks, is actually the top of a 21 foot high dam (similar to the one near the Smiths Falls Combined Lock), placed in the main channel of the Rideau River.