For years, officials in the Czech Republic had pushed a dam project to protect a river south of Prague, and the critically endangered species living in it. But the project, hamstrung by land negotiations, stalled.

In the meantime, a group of chisel-toothed mammals — renowned for their engineering skills and work ethic, and unencumbered by bureaucracy — decided to take on the task. The beavers of Prague simply built dams themselves.

The rodents’ fast work saved the local authorities some 1.2 million euros, according to a news release from the Nature Conservation Agency of the Czech Republic, a government agency responsible for conservation across the country. “Nature took its course,” Bohumil Fišer, the head of the Brdy Protected Landscape Area, where the revitalization project was planned, said in the statement. The beavers, he added, had created the ideal environmental conditions “practically overnight.”

The project, on a former army site on the Klabava, a river about 40 miles southwest of Prague, the Czech capital, was drafted in 2018 and had a building permit, but had been delayed for years by negotiations over the land, which had been used as a military training grounds, Agence France-Presse reported on Tuesday. Officials had hoped to build a barrier to protect the river and its population of critically endangered crayfish from sediment and acidic water spilling over from two nearby ponds, A.F.P. reported.

The beavers began working before the excavators could even break ground. It was not immediately clear specifically when the dams were built and how long it took to build them.

The new wetland created by the dams covers nearly five acres, the conservation group said. It is twice as large as the area that the humans had planned, Agence France-Presse reported. “It’s full service,” Mr. Fišer told A.F.P. “Beavers are absolutely fantastic and when they are in an area where they can’t cause damage, they do a brilliant job.”

Despite their remarkable ability to construct dams, beavers often draw the ire of landowners and farmers for destroying trees, eating crops and flooding roads and fields. But in thinning a tree canopy, the rodents can often help to diversify an ecosystem by allowing in sunlight in so that other plant species can thrive, said Emily Fairfax, an assistant professor of ecology at the University of Minnesota.

“They’re fundamentally changing the way water and life moves through that landscape,” she said.

To build a dam, the beavers, whose weight as adults can range from about 40 to 80 pounds, begin by piling small stones across a river or stream, packing those stones in with mud, and repeating the process to construct a pond, which they then expand to become a wetland, Dr. Fairfax said.

They are motivated by their fear of predators: Beavers are adept swimmers and can hold their breath underwater for 15 minutes. On land, their ungainly waddle makes them easy prey. “They’re basically a big chicken nugget for predators,” which include bears, mountain lions and wolves, she said.

The Czech dam is not the first time the rodents have assisted in building a wetland. Beavers in California have helped to restore a floodplain about 30 miles northeast of Sacramento. In that case, the beavers’ work also helped local officials save money. “All they had to do was let the beavers be there,” Dr. Fairfax said. In other cases, beavers often did work that went unacknowledged. “We sort of have a blindness for beavers,” she said, noting that they were often considered a nuisance because of their alarming size and capacity to rapidly change the landscape.

“They’re powerful, they’re big, and they’re elusive,” Dr. Fairfax said, noting that, despite the beavers’ engineering prowess, they presented a challenge for conservation groups when planning restoration projects.

“Oftentimes we don’t want to allow the beavers to make the choices, because it’s hard to plan around that uncertainty; it’s hard to turn over control to a giant water rodent,” she said. “But that’s when beavers are at their best.”