Freshwater fish can now swim from Upper Yaphank Lake into the Carmans River to reproduce for the first time in about 300 years.

Brookhaven Town officials on Monday announced the completion of a new fish passage and reconstruction of a spillway that allows trout to travel between the two bodies of water.

The yearlong project was funded by a $650,000 state matching grant and included the town cleaning up the lake.

“The river will flow free so that fish will spawn and reproduce,” Brookhaven Supervisor Edward P. Romaine said at a news conference in Yaphank.

English settlers in the 1700s built water mills to grind grain and constructed two dams that have prevented fish from traveling upstream, town officials said.

“This is great for Brookhaven and its mightiest river, and we are undoing some of the damage done by the dams,” Romaine said.

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Trout will be able to swim along the upper part of the river and follow a stream through either the spillway or the passage to spawn each spring. The passage provides an additional ecological benefit by restoring the lake-river connection where more fish will populate the waters and attract fish-eating birds, officials said.

“It’s finally coming to fruition,” Bob Kessler, of the Coalition to Save the Yaphank Lakes, said of the project.

Town officials said the spillway also adds capacity to carry excess water and prevent future flooding, town officials said.

“Our environment is irreplaceable. I’m happy to say the state put some money in to move this forward,” said state Sen. Ken LaValle (R-Port Jefferson), who secured the grant.

Romaine said he is awaiting Suffolk County’s reconstruction of the county-owned Lower Yaphank Lake to allow fish to spawn there. “Suffolk County has budgeted the money to do it,” Romaine said.

Chart Guthrie, regional fisheries manager with the New York State Department of Environmental Conservation, said the agency has been working with the county on plans for that lake and reviewing dam safety and freshwater wetland permits. Suffolk officials did not respond to a request for comment.