Yukon First Nations and environmentalists are celebrating a Supreme Court of Canada ruling they say will protect the environment and wildlife in a vast and largely untouched region of northern Yukon, as well as First Nations land use rights enshrined in treaties.

The unanimous high court ruling on Friday overturned an appeal court decision that rolled back years of planning for a massive swath of sub-Arctic wilderness and ended a five-year legal battle with the Yukon government.

The top court ruled the territorial government did not have the authority to make wholesale changes to a protection plan for the Peel watershed that was proposed by an independent commission after years of consultations.

“For several years, we’ve been on long, twisting journey to hold the Yukon government accountable for promises made during the lands claim process and to protect an area valuable to our First Nations and so many other Yukoners,” said Chief Roberta Joseph of the Tr'ondëk Hwëch'in during a press conference in Ottawa.

Christina McDonald, executive director of the Yukon Conservation Society, said the ruling “stopped a government that had gone off the rails.”

Chris Rider, executive director of the Yukon chapter of the Canadian Parks and Wilderness Society said the ruling culminates 20 years of work for many people and a vision where his future grandchildren will be able to hike, camp and paddle in wild places.

“I want them to be able to live in a world where bears, caribou, wolves and lynx exist in wild spaces, not just in zoos.”

The Supreme Court rejected a ruling by the Yukon court of appeal that would have sent the parties back to consultations on the watershed. The appeal court “improperly inserted itself into the heart of the ongoing treaty relationship between Yukon and the First Nations. Yukon must bear the consequences of its failure to diligently advance its interests and exercise its right to properly propose modifications related to access and development to the recommended plan. It cannot use these proceedings to obtain another opportunity to exercise a right it chose not to exercise at the appropriate time.”

The decision has been highly anticipated in Yukon, where the issue has been a thorn in government-First Nations relations.

It means the process returns to the later stage of the process, in which Yukon can approve, reject or modify the final plan. The Supreme Court clarified that any modifications at this stage must be based on those proposed earlier in the process or respond to changing circumstances, adding that modifications are by definition "minor or partial changes."

The vast Peel watershed is almost the size of New Brunswick - approximately 68,000 square kilometres – and makes up 14 per cent of the area of Yukon. It is the traditional territory of the Tr'ondëk Hwëch'in, Na-Cho Nyäk Dün, and Vuntut Gwitchin First Nations. It has no permanent residents and few roads. Its rugged mountains and taiga forests are home to a wide range of fish, wildlife and plant species.

There is potential to expand wilderness tourism, big-game outfitting and trapping, along with growing interest in mineral and oil-and-gas exploration. The region has many active quartz mines and large deposits of iron and coal. There are also thought to be significant gas reserves.

The dispute centred on Yukon’s land use planning process for the watershed after an independent commission recommended a plan in 2009 that would have protected about 80 per cent of the area from development. The Yukon government rejected that plan, putting forward its own in 2012 that would protect just 30 per cent of the watershed.

First Nations and environmental groups took the government to court, arguing the territorial government had breached its treaty obligations under a 1993 deal called the Umbrella Final Agreement. The UFA requires a collaborative process for land use planning.

The lower courts largely sided with the First Nations and environmental groups but the court battles went on over how to resume the planning process. Lawyers for the Yukon government argued the process should start again with consultations but First Nations and environmental groups want the commission’s plan to be enacted.

Friday, the top court held that the government had the right to make modifications but Yukon’s changes to the recommended plan were so significant as to effectively reject it. Yukon “did not have the authority” to do that under the terms of the treaty, the court ruled.

“Yukon’s changes to the Final Recommended Plan did not respect the land use planning process in the Final Agreements and its conduct was not becoming of the honour of the Crown.”

The court said that judicial review of the implementation of modern treaties “should simply assess whether the challenged decision is legal, rather than closely supervise the conduct of the parties at each stage of the treaty relationship,” reads the ruling. “Reconciliation often demands judicial forbearance. Courts should generally leave space for the parties to govern together and work out their differences. However, judicial forbearance should not come at the expense of adequate scrutiny of Crown conduct to ensure constitutional compliance.”

With files from The Canadian Press