EAST PROVIDENCE — Not so long ago, the last of three fish ladders on the Ten Mile River was built, capping years of work costing millions of dollars to help herring get over dams that had blocked access to historic spawning grounds for more than a century.

But it turns out that it wasn’t just man-made obstructions that were keeping the fish from Turner Reservoir during their annual upriver migration each spring. A natural bedrock ledge just below the Hunt’s Mill Pond dam has also been stymieing progress of the little fish.

The conditions have to be just right for river herring to swim over the three-foot-high shelf. When waters are high, they churn up rapids that push the fish downriver. When they’re low, the leap necessary to clear the exposed stone is beyond the ability of most herring.

To be sure, tens of thousands of herring have been able to make the journey up the river since the completion in 2015 of a series of projects totaling $7.7 million that was led by the U.S. Army Corps of Engineers with support from The Nature Conservancy, the state Department of Environmental Management, the City of East Providence and Save The Bay.

After getting through the fish ladder at Omega Pond Dam — the one that opened five years ago — they reach the ladder that takes them above the dam at Hunt’s Mill (finished in 2011) and on their way, finally, to the Turner Reservoir ladder (built in 2012).

But it’s uncertain how many fish haven’t been able to make it over the ledge. And even for those that were able to, the fight over the rocky obstacle would have been draining and possibly made them less likely to reproduce, said Patrick McGee, a migratory fish expert with the Rhode Island DEM.

“There’s no doubt that under absolutely ideal conditions, fish are passing, but it’s not always ideal conditions,” he said. “The idea is to get the fish up through the river so they expend the least amount of energy possible.”

If the newly restored herring run is to reach its full potential as one of the biggest in the state — which means bringing annual numbers of fish as high as 200,000 — it will mean doing everything possible to ease the path upriver for a species that plays a critical role in the Narragansett Bay ecosystem.

So, earlier this month, an important step was taken to help solve the problem posed by the ledge. A crew with SumCo Eco-Contracting, of Peabody, Massachusetts, created four staging pools in the route up one side. The pools, made using 30 sand-filled “super sacks” weighing 3,400 pounds each, will act like a staircase that the fish should be able to swim up with ease.

If all goes well, the sacks will stay in place for no more than a year. They will be tested by next spring’s returning fish, and sometime afterward a set of concrete weirs will replace them. The Nature Conservancy managed the design and installation of the model and the Army Corps will build the permanent structure.

It’s not usual for designers of these sorts of projects to test physical models in a river, but the structure of the river bottom and the rapids presented difficulties for digital modeling. Nobody wanted to take a chance with the project, so the temporary model was built at a cost of $53,000.

Why is so much money and time being invested in aiding a fish that sits near the bottom of the marine food chain? Well, precisely because of its lowly position. River herring play an outsized role in the ecosystem because many bigger animals rely on them as food, from striped bass and bluefish to ospreys and herons.

“The big picture is you’re helping the system as a whole,” said McGee, a principal biologist with the DEM. “We’d like to replenish those herring stocks to be the foundation.”

The Ten Mile River, which flows into the Seekonk River and Narragansett Bay, isn’t the only place in Rhode Island where organizations have been working to restore herring runs that were impeded by the construction of dams during the Industrial Revolution. The Nature Conservancy and other partners, for example, have built a series of projects on the Pawcatuck River, which stretches from South Kingstown to Westerly, and on the Annaquatucket River, in North Kingstown.

Bryan Sojkowski, a U.S. Fish and Wildlife Service fish passage engineer who works all along the Northeast and Mid-Atlantic coasts, said that many states are working to bring back river herring and other migratory fish like American shad. But he said Rhode Island stands apart in the progress that’s been made and the speed with which projects get off the ground.

“As someone who’s passionate about the resource, I’d like to see things happen at a quicker pace, and Rhode Island is where it’s happening,” said Sojkowski, who is helping to design the new project on the Ten Mile River and has worked on other rivers across the state.

The next target could be the Blackstone River. It may seem unlikely because of the number of dams on the river and their height, but, although it would be expensive, it’s possible to bring herring back up the river, said John O’Brien, policy specialist with The Nature Conservancy.

“There’s strong interest,” he said. “Imagine what a boost that would be for forage in upper Narragansett Bay. It would be an amazing feat.”

As for the Ten Mile River, even before the fish ladders were built, a remnant population of herring continued to return, helped on their way by volunteers who manually lifted them over the dams.

Since construction wrapped up, the numbers of fish have steadily increased, from 10,000 in one year to a high last spring of 20,000 that were counted just above Hunt’s Mill dam.

Those involved in the efforts hope those numbers will continue to go up.

“It’s been a pretty good success story in a pretty short period of time,” McGee said.