The keystone arch dam at Jones falls is constructed of interlocking tapered vertical stones. The dam is one of the most spectacular engineering structure on the Rideau Canal, and is one of the first of its type in the world.
Originally designed to be 17 m (48 ft) in height, it was increased by 2.5 m (8 ft) at the bottom (to find solid bedrock in the river bed) and 2 m (7 ft) at the top (to prevent overflow). It extends 107 m (350 ft) across the gorge and was the tallest dam in North America when it was built.
It consists of three sections. First, a masonry face that is 8.5 m (27" 6") wide at the base, then, a clay puddle core for watertightness in the middle and lastly, earth fill on top, extending 39 m (127 ft) upstream. Like the locks, all the stones were carted from a quarry 10 km (6 mi) away.
The waste weir was constructed as an indispensable addition for the security of the dam. Originally, excess water was to overflow the dam but erosion at the base was feared. The weir was cut through very hard rock to provide a water control mechanism and is still used to discharge excess water and maintain levels in Sand Lake.
The old riverbed at the base of the dam supports a unique and fragile plant community. The protected southern exposure and fertile soil that is moistened by seepage from the dam supports mosses, liverworts, sedges, mint, touch-me-not, turtle head and Jack-in-the-pulpit.
The dam has often been called the "whispering dam" because its acoustics allow the sound of someone speaking at one end of the face to be heard at the other.