Portage Diversion

Background and Description

The Portage Diversion (sometimes referred to as the Assiniboine River Diversion) is a 29 km (18 mile) long channel located just west of Portage La Prairie that diverts water from the Assiniboine River northward into Lake Manitoba. The Portage Diversion is one of the major flood control infrastructure components that were recommended in the Report of the Royal Commission on Flood Cost Benefit (1958) (ZIP, 14 MB). Construction of the Portage Diversion started in 1965, and was completed in 1970 for a total cost of $20.5 million. The Portage Diversion was first put into operation in the spring of 1970. The Portage Diversion provides flood protection to the City of Winnipeg and to the communities along the Lower Assiniboine River, which include the Rural Municipalities of Portage La Prairie, Cartier, St. Francois Xavier and Headingley.

The Portage Diversion project includes the following components:

• a 29 km (18 mile) long diversion channel
• embankment dikes adjacent to the diversion channel
• two drop structures within the channel and an outlet drop structure at Lake Manitoba to reduce flow velocities in the channel
• an Inlet Control Structure at the entrance of the diversion channel to control flow entering into the diversion.
• a River Control Structure that acts as a dam and spillway along the Assiniboine River downstream of the diversion channel
• a reservoir upstream of the River Control Structure (the Portage Reservoir).

The Portage Diversion was designed to convey up to 708 cms (25,000 cfs) from the Assiniboine River when the Portage Reservoir is at full supply level of 234.4 m (769.0 ft). The last five kilometers of the diversion channel passes through Delta Marsh and this portion of the diversion channel has a lower capacity of only 425 cms (15,000 cfs). Any flows in excess of 425 cms (15,000 cfs) spill laterally from the channel into Delta Marsh through a section of the west embankment.

Operation of the Portage Diversion is guided by the following objectives:

• to provide maximum benefits to the City of Winnipeg and areas along the Assiniboine River downstream of Portage la Prairie.
• to minimize ice jams forming along the Lower Assiniboine River.
• not to increase the water level in Lake Manitoba beyond the maximum regulated level of 247.763 m (812.87 feet), if possible.
• prevent overtopping of the failsafe section in the Portage Diversion, if possible.

Portage Diversion Channel Drop Structure	Inlet Control Structure
River Control Structure	Portage Reservoir (facing west)