The yet-to-be-built Keeyask generating station will flood about 45 square kilometres of land when it comes into service in 2021, an environmental hearing heard Thursday.
That's less than was originally forecast because of modifications to the dam's design, but it's just the starting point of more widespread flooding in future years due to erosion caused by higher reservoir water levels upstream of the dam.
The issue of potential flooding was raised by Manitoba Wildlands director Gaile Whelan Enns during questioning of a Hydro panel on whether Hydro had learned from past mistakes in its hydroelectric development in the province's north.
Enns said environmental impact statement documents show the full extent of flooding on the Nelson River due to Keeyask won't be known until at least eight years after the powerhouse goes into operation.
Vicky Cole, manager of major projects assessment and licensing for the Crown utility, told the Clean Environment Commission all parties are aware of predicted wider flooding. Keeyask is being developed in partnership with four First Nations most affected by the new dam -- Tataskweyak Cree Nation, Fox Lake, War Lake and York Factory.
Hydro officials said more widespread flooding, mostly caused by the erosion of peatlands along the Nelson River, could eventually take in an estimated eight-square-kilometre area. By comparison, flooding caused by the Kettle generating station following its construction in 1974 flooded a 220-square-kilometre area along the Nelson River.
Hydro said during the first 30 years of operation, the reservoir will expand in area by about seven to eight square kilometres because of shoreline erosion and peatland disintegration.
Cole said because of local concerns, Hydro changed the dam's design to reduce the amount of flooding but still keep the 695-megawatt facility economically feasible.
"The communities that are most affected by this development have been engaged," Cole said. "They've been engaged in helping us design mitigation to ensure that it's appropriate and minimize the adverse effects of the project."
The CEC is hearing evidence for the next six weeks on the environmental impact of Keeyask to decide whether or not to recommend the province issue an environmental licence.
The feasibility of the dam is also to be studied by the Public Utilities Board in the new year in what's called a Needs For and Alternatives To review.
Manitoba Hydro and the province say the $6.2-billion dam is needed to meet growing domestic demand for electricity and to meet future export commitments to the United States.
On the same day the CEC started hearing evidence in Winnipeg Monday, Minnesota Power filed paperwork with the state regulator to get approval to build the Great Northern Transmission Line from the Manitoba border to Duluth. The line is to be co-owned by Minnesota Power and Manitoba Hydro.