The region's intact freshwater habitats are among the rarest ecosystems on Earth, says Jim Pojar, a biologist and forest ecologist who signed the letter. The downstream reaches of the transboundary rivers are "salmon landscapes" where marine nutrients carried inland by big runs of salmon nourish virtually everything else, he says. These ecosystems will become even more important with climate change, because water temperatures to the south are expected to rise above the levels that salmon can tolerate. "Rivers like the Taku, Stikine, Unuk and Iskut may be one of the last climate change sanctuaries for salmon," Pojar says.

But the region is so remote, and the human population so small, it's difficult to draw attention to the potential impacts of the mining. Last August, I visited Hyder, an Alaskan town with roughly a hundred residents. It's at the end of a natural inlet called the Portland Canal, and its neighboring town, Stewart, B.C., which has about 500 residents, would serve as a port for some of the new mines.

Hyder and Stewart are remnants of a mining boom that reached its zenith decades ago. Today there are scores of abandoned mines in the mountains, some in various phases of redevelopment, and numerous tailings impoundments. Runoff from the old mines -- which were much smaller in size and impact than modern open-pit mines -- has "likely" degraded water quality in the Salmon River, which empties into the inlet near Hyder, according to Canadian environmental agencies. That pollution includes dissolved sulfate and high turbidity. Farther north near Juneau, the Tulsequah Chief Mine, on a B.C. tributary of the transboundary Taku River, has been leaching acid into the salmon-rich Alaska stretch of the river ever since a Canadian company began mining the site in the 1950s -- despite the Canadian government's repeated orders to a series of numerous owners that it be cleaned up.

Tom Holmes is among the Hyder locals who are worried about the new mining rush. When we meet in the Temptations Bakery and Deli on Stewart's main drag, Holmes describes how he got here: As a young man, he worked as a coal miner in his native Pennsylvania, but "had enough of that pretty quickly," then drifted west to Oregon and finally up to Alaska. He's in his early 60s, with a ponytail protruding from the back of his ratty leather Harley Davidson hat. He says he and other locals depend on coho salmon.

"It's a fragile wild run, and I rely on it for food in the winter," says Holmes, who fills his freezer with his catch each year. The value of salmon in the local economy transcends its importance as winter food. A spawning channel built on Fish Creek close to the towns attracts 30,000 tourists a year; they hope to see bears and even gray wolves feasting on returning spawners. On the day I visit, hundreds of ragged chum salmon are struggling up the creek. The 50 or so visiting bird watchers from Canada and the Lower 48 don't seem disappointed by the day's lack of bears; instead, they eagerly scout the trees for the birds that scavenge the carcasses.

Hyder and Stewart, like the rest of Alaska and B.C., share a profound, interrelated dependence on salmon, wilderness, and tourism. Yet even Alaskans like Holmes appear resigned to what's coming over the border. "People dangle jobs in front of you," he says. "Most don't think about anything else, other than a house and a truck."

The tribal communities on both sides of the border may have the best chance to slow and moderate the mining rush. That was apparent in the fall of 2011, when Seabridge Gold representatives came to Ketchikan, Alaska, for a public information session. Rob Sanderson Jr. was waiting. As the vice president of the Ketchikan Indian Community and vice chair of the Tlingit-Haida Central Council -- organizations that represent more than 28,000 southeast Alaska Indians of Tlingit, Haida and Tsimshian heritage -- Sanderson wondered what would happen once the minerals were exhausted. "Just how do you plan on paying for ... protecting these dams after the mines are done after 52 years, and all of this stuff (waste rock and tailings and a soup of pollution) is backed up there?"

Sanderson says at least five southeast Alaska communities are located near the mouth of the Unuk River, downstream from the proposed KSM Mine site -- most with significant Native populations that rely on salmon from the river, as well as herring and oolichan from the river's mouth, and shrimp, crab and groundfish in the nearby marine waters.

Oolichan are of particular concern. The processed "grease" from these small, oil-rich anadromous fish has been a valuable commodity across the Northwest for millennia. Unuk River oolichan runs have been critically depressed since 2000, when the Alaska Department of Fish and Game closed the fishery. There are numerous theories about what's causing the decline: everything from warmer water and ocean acidification to pollution and by-catch by commercial fisheries. Meanwhile, king salmon runs -- the most important subsistence and commercial species on the Unuk River -- have collapsed. (Both fish disasters are also being experienced in other parts of Alaska.)

The Unuk's five salmon species are estimated to be worth $1 billion to Alaska's economy, effectively employing 7,000 people in fishing, processing and tourism. The KSM Mine would also threaten the neighboring Nass River system, which is much larger and even richer in salmon; the tailings would be in that river's watershed, while the waste-rock dumps would be in the Unuk River's headwaters. Sanderson says Alaska tribes are also worried about salmon and potentially degraded water quality on rivers to the north -- the Taku and the Stikine, which flow to the coast around Juneau and Wrangell.

The Ketchikan Indian Community is also reaching out to First Nations in B.C., strengthening the bonds among transboundary Tsimshian, Tlingit and Haida groups. (Sanderson himself was born in Alaska, but his mother is a Haida from B.C.). They're also sounding the alarm about the oil tankers that could soon access Alberta tar sands crude from the proposed Enbridge Northern Gateway pipeline terminus at Kitimat. It's risky when tankers ply the narrow, turbulent shipping channels of the coast; the tribes haven't forgotten the massive 1989 Exxon Valdez oil spill to the north.

"Right now, the biggest threat is that people are not educated enough," says Sanderson. "I don't think our local fishermen throughout southeast Alaska have a clue about the magnitude of what is about to happen in our own backyard."

But some of B.C.'s First Nations are divided in their evaluation of the risks and benefits. In the Stikine River watershed, members of the Iskut First Nation (part of the larger Tahltan Nation) have aggressively opposed the Red Chris Mine, coal mining on Mount Klappan, and Shell's coalbed methane plans, all in their backyard. But the Tahltan Central Council, which represents all 5,000 Tahltan members on resource matters, has signed agreements with the B.C. government supporting the construction of the new power line and the Forrest Kerr hydropower project. Other B.C. First Nations have also supported the power line.

That's why the Dogwood Initiative, an environmental group based in B.C.'s capital, has stepped back from the power line issue despite concerns about the cumulative impacts of mining. Executive Director Will Horton says that "local folks aren't standing up and fighting (the power line), so we don't come in like colonial outsiders and tell local people what's in their interests."

In Canada, both the federal and B.C. governments have rarely said "no" to a proposed mine -- a history not that different from the U.S., where the 1872 General Mining Act still gives miners nearly free rein on hundreds of millions of acres of U.S. federal land.