Beavers are Britain's native aquatic engineers and their return to sites in Scotland and England is doing wonders for the local environment, write Nigel Willby & Alan Law: restoring wetlands, recreating natural river dynamics and ecology, filtering farm pollutants from water, and improving habitat for trout and other fish.In Knapdale, damming by beavers transformed a small pond into a wetland of a type and complexity probably unseen in Britain for centuries. On the Bamff estate on Tayside, we found that grazing by beavers trebled the number of wetland plants in 9 years.Beavers have recently made a tentative return to Britain.