The Baker Dam in Lower Mills. Photo by Chris Lovett.

When the Neponset River pours over the dam at Lower Mills, the stately gloss of a pond, sometimes smooth as a mirror, narrows to a raging torrent. One story below, thick braids of white water unravel in a seething froth, then a cold spray rising several feet into the air. The plunge in the river no longer powers the work of factories, but it still rumbles. The sound reverberates from 19th century brick walls on either side, shaking the ground with the force of nature and more than 370 years of history.

History has also been viewed as part of the future at Lower Mills at least since the late 1970s, when developers began converting old brick factory buildings into housing. Along with the housing came more public access to the Neponset for recreation, especially along the greenway between Central Avenue and Port Norfolk.

With plans for extending the greenway farther upstream, there's growing disagreement about whether to preserve the echoes of history in Dorchester and Hyde Park or to restore something more like the river's pre-industrial flow. The first mill at what used to be known as the lower falls dates from 1634. But, in the 21st century, as more people cruise the river in canoes and kayaks, or hope to extend possibilities for fishing, a dam that evokes history is also considered an obstacle.

To start planning for the Neponset, the Mass. Riverways Program and Division of Marine Fisheries presented new study results last Wednesday night (Jan. 9) at St. Gregory's gymnasium in Dorchester Lower Mills. The studies were about the river's main pollution concern-deposits of cancer-causing polychlorinated-biphenyls (PCBs)- and the alternatives for restoration.

The alternatives include the elimination of dams at Lower Mills and the former Tileston & Hollingsworth paper mill in Hyde Park. Other options are less radical. These include retaining the dams, replacing them with rock ramps, and building "ladders" or side-channels to let salt-water fish get around the dams to spawn upstream.

Consultants for the river restoration say eliminating the dams would have fewer drawbacks and more advantages in clearing passage for boating and fish. Boaters currently have to get around the dams on foot.

The current steel and concrete dam at Lower Mills dates from the late 1950s. Consultant James G. MacBroom says the bridge has no capability for flood control and, if retained, would have to be rebuilt in about 30 years, at a cost of $3.6 million.

"In general, dam removal is the cheapest alternative, basically because you're not building anything new," he told the meeting.

MacBroom estimated that removing the Lower Mills dam would cost $5 million, much of that for removal of sediment contaminated with PCBs. "The structure that's actually there," he said, "is not very historic."

There was a different view of the dam among abutters of the river from Dorchester, who were outnumbered at the meeting by people who use the river for boating,

"This is the second oldest dam in the United States," said Dorchester resident Victor Campbell.

Another resident, Richard O'Mara, says he considers the Lower Mills dam a "historic structure."

"We're not looking for the least expensive way out here," he said. "History is something that can't be replaced once it's removed."

Also opposing removal of the dam is the chair of the board of trustees for the Baker Square Condominiums, David Colton. At the meeting he called for a closer look at the health risks posed by PCBs.

The new study by the US Geological Survey shows higher concentrations of PCBs in sediment on the upstream side of the dams. The highest concentration was near the dam in Hyde Park.

Near both dams, the concentrations were high enough for state law to require removal and disposal of the PCBS if they were to be exposed by changes in the river. It's expected removal of dams would leave more PCBs exposed by receding mill ponds, but MacBroom says even with dams in place, some PCBs are slipping downstream.

Two officials at the meeting-state Rep. Linda Dorcena Forry (D-Dorchester) and Mayor Menino's Chief of Environmental & Energy Services, James Hunt III, spoke in support of improvements along the river but stopped short of taking a position on the dams. Hunt called the restoration project an opportunity for a "renaissance" around the river, but he said planning discussions should include "other key stakeholders."

State officials say they're only starting a process for reaching a consensus before going ahead with a restoration plan. And the advocacy group, the Neponset River Watershed Association, is asking for members to serve on a new citizen advisory committee.

"I have no illusions that we can come up with an alternative that everyone's enthusiastic about," said the association's director, Ian Cooke. "But maybe we can come up with an alternative that everyone can live with."