As if on cue, a great blue heron soared low over White Rock Dam on the Pawcatuck River Friday afternoon, just in time to hear John O'Brien explain the plan to tear down the concrete barrier and reopen that stretch of the waterway to some of the big bird's favorite meals - herring, eel and shad.

"This is the first major obstacle for fish swimming up the Pawcatuck River," said O'Brien, partnership specialist with the Rhode Island office of the Nature Conservancy, while standing on the Pawcatuck side of the dam. "We want to address fish passage and to reduce the risk of flooding."

Signs of the dam's disrepair are obvious - a large tree grows out of the center of the structure, and water pours through two openings in a channel that divert the natural flow. Since 1770, a dam at the site has restricted flows and passage for migrating fish at the juncture between Pawcatuck, off Alice Court, and the end of White Rock Road in Westerly, just after the river takes a sharp bend from the east and turns south toward Little Narragansett Bay. The latest version of a series of structures first built to provide water power for textile mills along the river, the current concrete structure dates to about 1940 and has long outlived its purpose.

"It's only a matter of time before that dam fails," Nils Wilberg, project manager for the engineering firm Fuss & O'Neill, told about 50 people at an informational meeting in Westerly Wednesday. "Our choice is to remove it in a controlled manner to reduce the risk."

The conservancy, in partnership with the U.S. Fish & Wildlife Service, has secured a $2.3 million federal grant to remove the dam, the third such project on the river in recent years. At the meeting, Wilberg met with a supportive audience as he described the benefits to wildlife of returning the river to its natural flows and to people and property prone to flooding when high water backs up behind the dam. That happened most dramatically in the spring of 2010 and again in Hurricane Irene in 2011 and Superstorm Sandy in 2012. A dam failure would release a powerful wave of water capable of causing more damage than any of the previous floods.

O'Brien said the grant is part of Sandy relief funds that are being directed to projects that will make communities more able to handle future severe storms.

"The grant is to become more resilient to flooding in the future," he said.

The White Rock Trust that owns the dam and Griswold Textile Print downstream support the project and are negotiating with the conservancy for the easements and other legal agreements needed, said Attorney Sarah Moriarty, a trustee.

The conservancy and the Fish & Wildlife Service are working with the Wood Pawcatuck Watershed Association, the Rhode Island Department of Environmental Management, the Connecticut Department of Energy and Environmental Protection and the National Oceanic and Atmospheric Administration on the project. O'Brien said permitting will be especially complex because the border between Connecticut and Rhode Island lies in the middle of the dam, but that means the conservancy, as a third-party non-governmental group, is perhaps best suited to taking on such a project.

"The amount of bureaucracy around the removal is doubled," he said, "but that's the value of the conservancy doing this."

Plans call for permit applications to be submitted in January, and for construction to begin in July, Wilberg said. The work would be completed by the fall, with final restoration of the site taking place in the spring of 2016.

Now, fish that swim upstream to spawn either get stuck where the natural riverbed dead-ends, or become exhausted trying to swim through the artificial channel built to control flow for the mill, because of the high velocity of the water through the narrowed passage.

"Many fish don't make it," Wilberg said.

Once the dam is removed, the artificial channel would be sealed off with a berm so water would flow back along its natural course more gradually.

"It's going to be a natural system," he said. "We're looking to restore the wetlands systems there to what they were before the first dam was constructed. It's going to have some meanders and boulders and some pools where fish can hang out and catch their breath."

j.benson@theday.com

Twitter: @BensonJudy