U.S. officials say building a $57 million concrete dam and fish bypass channel along Montana's Yellowstone River offers the cheapest way to help an endangered species that dates to the time of dinosaurs.

The U.S Army Corps and Interior Department are proposing the irrigation dam on the lower Yellowstone near the North Dakota border.

That's where aging pallid sturgeon have been trapped for decades downstream of their spawning grounds.

Environmentalists say the dam could doom the fish and there's no proof the bypass channel would work.

They want the government to remove an existing man-made rock weir on the river. The weir diverts water for 58,000 acres of cropland.

A government study published Thursday said removing the weir and using pumps to get water would cost $138 million or more.