It would protect certain areas crucial to wildlife migration, as well as important feeding grounds necessary for the balance of the fragile Arctic ecosystem. Republicans have promised to overturn it as it "squanders" drilling opportunities.
"An early proposal for the Dakota Access Pipeline called for it to cross the Missouri River north of Bismarck, but one reason that route was rejected was its potential threat to Bismarck’s water supply." So they sent it through the rez instead.
"It took me, a white non-native woman being arrested... to bring this cause to many people’s attention." Article about her arrest while protesting the Dakota Access Pipeline by Shailene Woodley for TIME, 20 Oct 2016.