Troubled by a history of uranium mining, the Sahtu Dene of Great Bear Lake, NWT, create North America's largest biosphere reserve to protect its pristine waters for all of humanity. At the edge of the Arctic Circle, Sahtu Lake (Great Bear Lake) is earth's largest remaining fully functioning cold fresh-water ecosystem. This lake was spared the environmental impacts of colonization with no commercial logging, fishing, mining, or hunting, until in 1942 it became one of the first sources of uranium ore in North America. Until 1960 mine workers extracted uranium not knowing about its toxicity. Some 740,000 tonnes of tailings were dumped into the lake. Residential schools robbed children of their culture, and hunting was outlawed for the Dene. In the Dene legend of The Waterheart, a man finds Tudza, a heart that gives life to the lake. Sahtuot'ine negotiations led to the 1993 Sahtu Dene and Métis Land Claim Agreement, and self-government in 2016. The Great Bear Watershed was declared a UN Biosphere Reserve, the first ever created by an indigenous community. Déline is a fly-in community with expensive groceries; "country foods" are critical to the health, well-being and independence of the Sahtu. Traditional environmental management practices govern their ways, and elders are teaching ancestral skills, spiritual respect for the ecosystem and their native language to the youth.   Stewardship of the lake has its challenges. Sportfishing depleted the lake trout population so catch and release sports fishing has taken its place.  When fish are dissected for scientific study, they are used later as food. Climate change is clearly affecting the lake ecosystem, but the unique interspecies diversity of its fish indicates that they might adapt. Cleanup of former mine sites is critical to the lake's health. What they have accomplished here is impressive, and trained Guardians will truly know the land and understand the importance of the water.