The animals constructed the dam at the Holnicote estate, near Minehead, following river restoration work by the National Trust.
Footage captured on wildlife cameras shows the beavers gnawing on trees and collecting vegetation to create a dam across channels that run through the Somerset estate.
Praised as "ecosystem engineers" by rangers - the animals have created an "instant wetland" just 9 months after being introduced to restore streams and reduce flooding.
Ten months ago, the semi-aquatic rodents were the first to be released into the wild by the trust in its 125-year history.
In the 16th century, beavers became extinct in Britain after being hunted for their meat, fur and scent glands.
Dams like these allow for deep pools of water which offer animals shelter from predators and a place to store food, helping communities living nearby and wildlife in the area.
The ponds and channels also help prevent flooding through slowing, storing and filtering water as it flows downstream.
Ben Eardley, project manager at the National Trust said: "It might look modest, but this beaver dam is incredibly special - it's the first to appear on Exmoor for almost half a millennium and marks a step change in how we manage the landscape".
He added: "We've already spotted kingfishers at the site, and over time, as the beavers extend their network of dams and pools, we should see increased opportunities for other wildlife, including amphibians, insects, bats and birds.
"The recent rain we've had is a reminder of the significant role beavers can play in engineering the landscape.
"As we face into the effects of climate change and more frequent extreme weather events, natural interventions like this need to be part of the solution."
The beavers at Holnicote were relocated from wild populations on the River Tay catchment in Scotland, under licence from Scottish Natural Heritage.
They settled into a specially-built 2.7-acre enclosure on the estate in January and have been monitored by National Trust and Exeter University staff and volunteers since.