The province's environment minister has approved an environmental assessment for a highway twinning project near Windsor, N.S., a move that is drawing the ire of a number of environmental groups.

Iain Rankin gave the green light on Tuesday for the Department of Transportation and Infrastructure Renewal to proceed with twinning Highway 101 between Exit 5 at Three Mile Plains and Exit 7 in Falmouth, a distance of about 9.5 kilometres.

The approval comes with many of the standard conditions for such projects, including that the department monitor the effects on wetlands, erosion, sedimentation, water supply, groundwater, air quality, dust, noise, fish, flora, fauna and wildlife.

But the approval also orders the department to tell the public, and specifically Mi'kmaq people, why it chose to keep the causeway at the Avon River instead of building a bridge.

Causeway vs. bridge

The causeway has been a source of tension for some in the area.

While it helps protect the nearby communities of Windsor and Falmouth from flooding, some residents say the gates that control the flow of the water through the causeway have led to an accumulation of mud and siltation upstream and prevent fish from passing through.

Part of the twinning project will involve upgrades to the existing tidal gate structure, or aboiteau, at the Avon River causeway.

The causeway itself will be elevated about one metre and widened to six lanes to accommodate safe interchanges for exits 6 and 7.

The Avon Causeway helps protect the nearby town of Windsor, N.S., from flooding. (CBC)

Some residents had hoped the causeway would be replaced by a bridge if twinning was approved.

Joshua McNeely, the oceans management adviser for the Maritime Aboriginal Peoples Council, said a bridge would have likely been the most environmentally friendly solution.

McNeilly said he's concerned about the project's impact on nearby wetlands. The saltwater marsh near the causeway is a significant wetland and important bird habitat.

"There's obviously a balance that's needed between providing the infrastructure that Nova Scotians need for a safe highway, but at the same time balancing that with the needs of the wildlife and that important wetland," he said.

A spokesperson for the department wrote in an emailed statement that the causeway was determined to be the most cost-effective way of addressing flooding, fish passage and highway infrastructure needs.

Consultation concerns

McNeely also said the department has not consulted with the council on the project.

"We have concerns and [the department] hasn't really been forthcoming, at least to us," he said. "We've asked for consultation, we've asked for them to come down and explain the project and thus far we've been ignored."

As part of the environmental approval process, the department did consult with some Mi'kmaq groups, the environmental assessment registration document says.

Sonja Wood, a member of Friends of the Avon River, said the environmental approval was "disheartening." She said the group has sought legal advice and has notified the relevant government departments that it would like to see the approval reversed.

The causeway and aboiteau have prevented the endangered wild Atlantic salmon from getting to its habitat, said Wood. She wants to see at least 150 metres of the causeway opened to allow "free tidal flow" in the area. Wood also wants the federal Department of Fisheries and Oceans to step in to explore the effect of the project on the salmon.

'Sloppy work'

Wood is also concerned that the environmental assessment documents don't spell out how the aboiteau will be affected by the twinning project.

"To this day, I have not seen any plans for upgrades to the existing aboiteau. They don't even have this work done," said Wood. "It's just sloppy, sloppy work and it's unprofessional."

The environmental assessment document says the design of the upgrade is still unknown, but notes that staff will consult with the DFO on how to allow adequate fish passage. The province's approval requires the department to provide a detailed design of the aboiteau and an analysis of other options for fish passage.

A detailed design for the project is expected to be completed in 2018, but some construction could begin this fall. Construction is expected to take about five years.