Portage Diversion

Portage Spillway sending water down the Assiniboine River towards Winnipeg

Portage Diversion operating at about maximum designed channel capacity (25,000 cu ft/s (710 m³/s)) heading towards Lake Manitoba

Portage Diversion operating at expanded capacity at about 33,000 cu ft/s (930 m³/s) heading towards Lake Manitoba

The Portage Diversion (49°56′48″N 98°20′06″W) (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 Portage Diversion emptying into Lake Manitoba near Delta Beach at about maximum channel capacity of 25,000 cu ft/s (710 m³/s)

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.[1],[2]

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.

Effects on Lake Manitoba

When the Portage Diversion is opened, it sends water intended for Lake Winnipeg, a freshwater lake, into the slightly saline Lake Manitoba. This has a dramatic effect on Lake Manitoba water quality. In 2009 alone, when the Portage Diversion was open for just 27 days, the Portage Diversion itself contributed 99 percent of the suspended sediments, 60 percent of the nitrogen, and most importantly 93 percent of the phosphorus that flowed into Lake Manitoba that year. The result of the excess nutrients sent into Lake Manitoba is that eutrophication of the lake occurs. The result of eutrophication is that microorganisms such as cyanobacteria, a nitrogen fixing microbe can utilize the abudent supply of phosphorus to propagate itself. The end result is algal blooms which can result in the release of cyanotoxins in the case of cyanobacteria that are harmful to both humans, animals, and other marine life. The end result of all this leads to the deterioration of water quality, loss of aquatic life, disrutption of the ecosystem, and the real chance that the body of water could become a "dead lake" like Lake Erie in the 1960s.

The Portage Diversion has also contributed enough water at times to Lake Manitoba to cause the lake to rise significantly. The first major occurrence of this was during the 1976 flood on the Assiniboine River when 1,420,000 acre feet (1.75 km³) of water was diverted to Lake Manitoba, resulting in a 1.22 feet (0.37 m) direct rise in water levels. The flooding on Lake Manitoba in 2011 has been more severe as a result of the operation of the Portage Diversion. As of June 13, 2011 at least 3,000,000 acre feet (3.7 km³) (2.6 feet (0.79 m) rise) of water has been directed toward Lake Manitoba rather than Winnipeg and the Red River, this number already being more than double the volume of the 1976 flood. After considering the fact that without the existence of the Portage Diversion, some water would flow into Lake Manitoba near Delta Beach through oxbow lakes and channels which were at one time the original channels of the Assiniboine River when it flowed to Lake Manitoba naturally around 2000 years ago, it is estimated that the Portage Diversion contributed to about 2 feet (0.61 m) of a rise in water levels on Lake Manitoba, accounting for roughly two-thirds of the rise of the lake since the spring thaw.

See also

• Red River Floodway
• Shellmouth Reservoir

External links

• Flood control works