From the air, however, the mines fall away quickly. Skimming low over the river, startling a young moose that was fording a narrow channel, a government biologist named Preston McEachern and I veered northwest toward the Birch Mountains, over vast expanses of scarcely disturbed forest. The Canadian boreal forest covers two million square miles, of which around 75 percent remains undeveloped. The oil sands mines have so far converted over 150 square miles—a hundredth of a percent of the total area—into dust, dirt, and tailings ponds. Expansion of in situ extraction could affect a much larger area. At Suncor's Firebag facility, northeast of the Millennium mine, the forest has not been razed, but it has been dissected by roads and pipelines that service a checkerboard of large clearings, in each of which Suncor extracts deeply buried bitumen through a cluster of wells. Environmentalists and wildlife biolo­gists worry that the widening fragmentation of the forest, by timber as well as mineral companies, endangers the woodland caribou and other animals. "The boreal forest as we know it could be gone in a generation without major policy changes," says Steve Kallick, director of the Pew Boreal Campaign, which aims to protect 50 percent of the forest.

McEachern, who works for Alberta Environment, a provincial agency, says the tailings ponds are his top concern. The mines dump waste­water in the ponds, he explains, because they are not allowed to dump waste into the Athabasca, and because they need to reuse the water. As the thick, brown slurry gushes from the discharge pipes, the sand quickly settles out, building the dike that retains the pond; the residual bitumen floats to the top. The fine clay and silt particles, though, take several years to settle, and when they do, they produce a yogurt-like goop—the technical term is "mature fine tailings"—that is contaminated with toxic chemicals such as naphthenic acid and polycyclic aromatic hydrocarbons (PAH) and would take centuries to dry out on its own. Under the terms of their licenses, the mines are required to reclaim it somehow, but they have been missing their deadlines and still have not fully reclaimed a single pond.

In the oldest and most notorious one, Suncor's Pond 1, the sludge is perched high above the river, held back by a dike of compacted sand that rises more than 300 feet from the valley floor and is studded with pine trees. The dike has leaked in the past, and in 2007 a modeling study done by hydrogeologists at the University of Waterloo estimated that 45,000 gallons a day of contaminated water could be reaching the river. Suncor is now in the process of reclaiming Pond 1, piping some tailings to another pond, and replacing them with gypsum to consolidate the tailings. By 2010, the company says, the surface will be solid enough to plant trees on. Last summer it was still a blot of beige mud streaked with black bitumen and dotted with orange plastic scarecrows that are supposed to dissuade birds from landing and killing themselves.