The Rideau Canal opened in 1832, but the bulkhead dam at Watson’s Mill was not built until 1858. This raises the question of how the canal worked in the Manotick – Long Island area in the intervening years.

In fact Colonel By’s design of the canal in the Manotick – Long Island area did not contain a dam at the Watson’s Mill site. He used weirs elsewhere.

A weir is a low head dam used to increase the depth of water in a channel for some purpose. For example a weir could be used to create a mill pond to run a water wheel or yield a section of river navigable. Its not clear when a weir becomes high enough to become a dam.

Before the canal the water flow in the Manotick area was different than now. There were 3 channels instead of two, and there were three islands instead of two. The islands were Long Island, Nicolls Island, and Little Island. The map below shows the area in 1831. (Map from “A History of Long Island, Manotick, 1827 to 1997 by Catherine Carroll & Barbara Humphreys.) Little Island existed because the water levels were higher in both the west branch and Mud Creek before the dam was built at the Watson’s Mill site.