Portage Diversion

The Portage Diversion (49°56′48″N 98°20′06″W / 49.94667°N 98.33500°W / 49.94667; -98.33500) (also known as the Assiniboine River Floodway) is a water control structure on the Assiniboine River near Portage la Prairie, Manitoba, Canada. The project was made as part of a larger attempt to prevent flooding in the Red River Valley. The Portage Diversion consists of two separate gates which divert some of the flow of water in the Assiniboine River to a 29 km long diversion channel that empties into Lake Manitoba near Delta Beach. This helps prevent flooding on the Assinboine down river from the diversion, including in Winnipeg, where the Assiniboine River meets the Red River.

During flood years such as spring 2011, inlet flows to the Portage Diversion control structure were measured at over 54,000 cu ft/s (1,500 m3/s). This amount of water would have disastrous effects if left to flood southern Manitoba. During the flood of spring 2011 the Portage Diversion handled roughly half the flow of Niagara Falls.

The Assiniboine River can handle flows up to approx 19,000 cubic feet per second (540 m3/s) before spilling over its banks. During years when the Assiniboine River Watershed achieves flows greater than 19,000cfs into the Assiniboine River the diversion gates are opened to allow the excess water to spill into lake Manitoba, at the same time the gates that feed the Assiniboine river can be closed to restrict the flow.

The diversion was originally designed to carry a volume of 25,000 cubic feet per second (710 m3/s). Under a state of emergency in early May 2011, Manitoba authorities did extensive work by raising the dikes and were preparing to send up to 34,000 cu ft/s (960 m3/s) down the diversion channel with bridges downstream being the determining factor in flow rate.,

The diversion was built at a cost of $20.5 million dollars in 1970. The diversion control dam is 35 feet (11 m) high and 1,400 feet (430 m) long and allows 14,600 acre foot (18,000,000 m3) storage.