Obama, ask a Legislator to introduce a bill that stated whereas the DAPL would be disastrous in the case of a rupture, and could pollute the drinking water for 4 states, it is so irresponsible as to be unlawful for it to be completed. The one way to force Congress to pass it, would be for you to make a public announcement that the vote would be made public immediately, and the votes of every single member of Congress would be available for every voter to see, in the press, TV/radio, online, & even posted here.
This strategy assumes the vast majority of Americans, like the rest of the world, really do want to see DAPL stopped, and that legislators, whether they retained their seats or not, would not like to be on very public record that they could have stopped it, but didn't.