The largest dam removal in U.S. history entered a critical phase this week, with the lowering of dammed reservoirs on the Klamath River.

On Thursday, the gate on a 16-foot-wide bypass tunnel at the base of Iron Gate dam, the lowest of those slated to be removed, was opened from a crack to 36 inches.

Amy Cordalis stood in the dawn chill to witness the first big surge as the gate was widened. She’s an attorney and Yurok Tribe member who has played a critical role in advocating for dam removal. As water poured through the tunnel, she could hear boulders rolling and tumbling. The water turned to dark chocolate milk as decades of pent-up sediment surged through.

“This is historic and life-changing,” Cordalis said. “And it means that the Yurok people have a future. It means the river has a future; the salmon have a future.”