Rubble from a demolished dam on the Patapsco River has moved to its new home on the bottom of the Chesapeake Bay, and it will soon be joined by a boatload of new neighbors — 4 million baby oysters.

For more than 100 years, Simkins Dam in Ellicott City prevented eels, herring and shad from migrating upriver before it was torn down last winter at the urging of environmentalists. In its more eco-friendly second life, the dam’s concrete will serve as the base for a new oyster reef near the mouth of the Chester River.

“It was a win-win situation for fish upstream and downstream,” said Stephanie Westby of the National Oceanic and Atmospheric Administration.

With natural reefs in the bay in short supply, artificial beds have become increasingly important in the effort to restore the oyster population, said Erik Zlokovitz, artificial reef coordinator for the Maryland Department of Natural Resources.

Zlokovitz said the department is placing a greater emphasis on developing new oyster habitats because of Gov. Martin O’Malley’s push to revive the oyster population. They are increasingly using materials like limestone and granite that can serve as the base for artificial reefs.

“You have to be really resourceful now to build an oyster reef,” said Tom Zolper, Maryland communications coordinator for the Chesapeake Bay Foundation.

The idea to reuse concrete from the 10-foot-high, 150-foot-wide Simkins Dam to create an oyster bed came from NOAA, which funded the dam demolition using stimulus funds from the Recovery and Reinvestment Act.

“Once the dam was torn down in Ellicott City and allowed the fish to pass through, we thought it was a good idea to make good on the rubble left over from that project and bring it over to make good on another project for fish,” said Karl Willey, who runs the Chesapeake Bay Foundation’s oyster program.

The Maryland Department of Natural Resources, NOAA and the Chesapeake Bay Foundation coordinated with state and federal agencies to develop the new reef site, which is off-limits to harvest.

The dam’s rubble, a mixture of concrete and granite, was cleaned and crushed to the correct size before it was deployed to the oyster bed last week.

The treated rubble was placed at the 2-acre oyster reef site using large water cannons that pushed the rubble off of a vessel onto the reef site.

“It’s a good, nice, flat, hard bottom where we can place the stone and build the reef,” Zlokovitz said.

About 2,800 cubic yards of concrete and granite were placed in the bay, creating a bed that ranges from 1-to-3-feet high across the site, he said.

“We’re trying to build up the three-dimensional profile on the bottom and create nooks and crannies for the oysters to attach to,” Zlokovitz said.

The water at the site is around 19-to-20-feet deep. The level of dissolved oxygen at the depth creates a good environment for the oysters to thrive, Zlokovitz said.

The Chesapeake Bay Foundation provided spat — baby oysters attached to oyster shells — that were grown at the foundation’s Oyster Restoration Center in Shady Side.

“Typically we can produce about 12 [million] to 14 million spat on shell a year,” Willey said. “This project will take about 4 million spat on shell that we produced this past summer.”

Two million of the baby oysters were placed on the reef Thursday from the foundation’s restoration vessel, The Patricia Campbell. The final 2 million were expected to be placed on Monday.

“Growing them in our nursery actually makes them stronger for better survival out here in the wild,” Willey said.

For Willey, the success of the reef goes beyond the growth of the oysters — he hopes to see it attract new life, fish, plants and other wildlife along with the oysters.

“The goal of this project is to … come back out here and do it again hopefully next year,” Willey said.