On Monday, a demolition crew will begin removing the Veazie Dam on the Penobscot River just above Bangor, Me. The Veazie is the lowest of the Penobscot dams and closest to the river’s mouth on the Maine coast. It is also critical to the entire Penobscot River watershed, which covers nearly a third of the state. Thanks to the work of the Penobscot River Restoration Trust and its partners, the lower river will be free-flowing once again, allowing the revival of a complex migratory ecosystem once teeming with fish working their way up from the sea.

This is the second major dam to be removed on the Penobscot, and it is part of a nationwide movement. One phase of this nation’s environmental history was the building of thousands of dams for irrigation and hydropower. But we are in a new phase in which many of those dams, which always alter, if not destroy, the native ecosystem, are coming down. According to American Rivers, a conservation group, some 1,100 dams have been removed nationwide in the last century, including 96 in the Northeast since 1999. In the West, dams have come down on the Elwha and White Salmon Rivers in Washington, and there have been discussions about removing dams on the Klamath near the California-Oregon border and on the lower Snake River in Washington.

The iconic species of the Penobscot is the Atlantic salmon, which used to run up the river to spawn in large numbers. Spawning will not resume until work on the upper Penobscot watershed is completed, including new salmon passages at upstream dams. But opening the lower river will immediately benefit other species, including striped bass, herring, sturgeon and smelt.

There is, however, a troubling note in the restoration of free-running river systems, east or west. Anadromous species live only part of their lives in freshwater rivers. Much of their time is spent at sea. So while it is imperative to keep river restoration going, it is no less important to protect their oceanic habitat as well. It takes both habitats to make a salmon or a sturgeon or a smelt.