The environmental impact statement submitted by the provincial utility omitted several species, including the endangered short-eared owl, the Canadian Environmental Assessment Agency panel wrote in a request for information earlier this month.
In some cases, the mitigation measures provided by Hydro didn't appear to have any link to the species in question, the panel suggested.
"Please revise the tables to include all species at risk ... and ensure that the links between species listed and the mitigation measures are clear ...," the panel asked.
For example, the impact statement proposes mitigation measure for one species of bat but not another, and suggests wetland nesting boxes for a bird that lives predominantly in agricultural fields.
Members also found that BC Hydro failed to provide — among thousands of pages of studies and consultant reports — information on the effect the project will have on loss of black bear dens and some traditional First Nations activities.
And the panel said Hydro included only past activities in assessing the cumulative impacts of the project, leaving out the impact of other current hydroelectric projects, forestry and oil and gas activities.
In a 331-page response posted on the CEAA website on Thursday, Hydro estimated that about 2,000 hectares of short-eared owl habitat will be lost if the dam is built on the Peace River.
The owl was added to a list of raptors that will see habitat loss because of the dam, including the boreal and great horned owls, the northern goshawk and the broad-winged hawk.
Measures to reduce the impact will include amphibian crossings for the endangered western toad, and building replacement wetland habitat, but the Crown agency admits most measures will reduce, "but not fully mitigate" the potential effects.
"While the project is likely to result in some significant adverse effects, those effects are justified by the need for and benefits, including the benefits to sustainable development, of the project," BC Hydro said in an April 2013 submission to the panel about its original impact statement.
Environmental review hearings will begin next month in northeastern B.C., with a decision expected by mid-2014. The province of B.C. has an agreement with the federal government for a co-operative environmental assessment that will stand for both governments.
The $7.9-billion dam would produce enough electricity to power 450,000 homes per year for 100 years. It would flood about 5,000 hectares of the Peace River Valley, creating an 83-kilometre, 9,300-hectare reservoir between Fort St. John and Hudson's Hope.
The panel also asked BC Hydro to provide more information on its justification for the project, citing media reports about a substantial rate increase on the horizon.
"Electricity prices are likely to rise rapidly in the next few years, even without the effect of Site C," the panel wrote.
It's possible — depending on the timing of the project, the cost and the effect of prices on consumer demand — that Site C could be unnecessary, the panel suggested.
"Without Site C, demand could outstrip supply; with it, demand could be so constrained as to make Site C unnecessary for a long time," the panel wrote.
Hydro has said the project will be absolutely necessary, and provided reports and documents dating back a decade to show the demand is — and will be — there.
If approved, the project would take seven years to construct and would begin producing electricity in 2022 or 2023.
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