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The State Department has concluded that TransCanada’s proposed Keystone XL pipeline would, for the most part, have negligible effects on wetlands, wildlife, and waterways. The judgment, the conclusion of the released on March 1, has sparked outrage among conservation groups, which maintain that the .

The 1,700-mile-long pipeline’s southward slither would begin in Canada’s tar sands and travel through six U.S. states to refineries on the Gulf Coast. In northern Alberta, huge swaths of boreal forest have already been cleared to mine bitumen and heavy oil. “[The boreal forest] is critical breeding ground for birds coming through the United States,” including the , an ̽����ѡ priority species, says Mike Daulton, ̽����ѡ’s vice president of government relations. , lynx, , and are suffering, too.

As the pipeline wends its way through Montana, South Dakota, and southward, it would cut through prairies home to vulnerable species, including and pronghorn antelope, and cross the Yellowstone, Missouri, and Red rivers. A spill could be devastating.

“The big deal is that tar-sand pipelines may be at risk to rupture because diluted bitumen is very corrosive,” says Peter LaFontaine, the National Wildlife Federation’s energy policy advocate. TransCanada says it uses state-of-the-art safety measures, but LaFontaine notes recent accidents along the company’s existing pipelines.

In —a major water source for agriculture and communities. TransCanada has rerouted the pipeline to bypass the state’s Sandhills region, migratory habitat for more than . Even so, points out Marian Langan, ̽����ѡ Nebraska’s executive director, “this is the main migration corridor for the Central Flyway.”

The , which runs through Oklahoma and Texas, was approved last year and is already being built. The pipeline, if completed, would carry about a million barrels a day to the Gulf, where it would be refined, (largely) exported, and burned—exacerbating global warming. Even the State Department report found that tar-sands oil is more greenhouse-gas-intensive than other fuels.

And if warming continues as predicted, ̽����ѡ’s models show, “people across large sections of the U.S. are likely to see declining bird diversity in their backyards, particularly during the summer,” says Justin Schuetz, director of conservation science. Vulnerable species include the chestnut-collared longspur, American oystercatcher, black rosy-finch, seaside sparrow, and yellow-billed magpie.

President Obama could make a final decision on the pipeline as early as this summer, although even if he kills it, Congress could try to force it through. Which means, says Daulton, that now is the time to lean on elected leaders.

A version of this story, called "Pipe Dream," ran in the May-June 2013 issue.