The Intake Diversion Dam in Montana along the Yellowstone River, a tributary of the Missouri, provides water to around 55,000 acres of farmland. But dams like this make it hard for the pallid sturgeon. And this is the last stronghold of sturgeon that have not interbred with another species of the fish. Stuck between this dam and the Fort Peck Dam to the northwest, the pallid sturgeon can no longer travel far enough up the fragmented rivers to ensure their eggs will make it to a healthy place to develop.

Normally, the fertilized eggs drop to the bottom of the river and are at the current’s mercy until they develop tiny tails, kind of like tadpoles, to propel them around. But the dinosaur fish usually can’t get past dams. So the eggs end up trapped in reservoirs like Lake Sakakawea, with a lot of sediment, a lot of bacteria and very little oxygen. There they suffocate and die.

“The headwaters of Lake Sakakawea is a dead zone,” said Christopher Guy, a biologist at Montana Cooperative Fishery Unit, who published a paper about the same thing happening around Fort Peck Dam last year.

The fate of these ancient fish rests in restoring their ability to swim freely on a long river. But how to make this happen is caught up in a legal dispute between government agencies, who want to build a passageway, and wildlife protection groups, who want to get rid of the Intake Diversion Dam altogether. Last year, biologists and conservationists got a judge to temporarily block dam construction while the agencies conducted a more thorough study of the options and their possible effects on the pallid sturgeon. Based on this assessment, federal agencies have put out a draft proposal that is open for public comment until July 28.