Photo: Water Vole sitting on bank edge (Photo by Peter Trimming 'I see no ships' via Wikimedia Commons)
Water voles are the largest of the three vole species we have in the UK measuring around 12-20cm. Water voles have a glossy, silky, yellowish-brown to dark brown coat of fur, a blunt nose, a rounded body and a long slightly hairy tail. Water voles are a secretive species but they do leave plenty of signs indicating their presence alongside a water body such as:
Photo: Water vole footprints imprinted into mud (photo by David Perez [Own work] [GFDL], via Wikimedia Commons)
It is a common to mistake water vole for other members of the rodent family such as other voles but in particular the brown rat (Rattus norvegicus). When compared to other voles, the water vole at 12-23cm is considerably larger than its cousins the Bank Vole (Myodes glareolus) at 9-11cm and Field Vole (Microtus agrestis) at 9-13cm in size.
It is more difficult to distinguish between water voles and the brown rat due to their similar size and cohabitation of the same environments. Whereas the water vole has glossy, silky, yellowish-brown to dark brown coat of fur, a blunt nose, a rounded body and a long slightly hairy tail; the brown rat is brown or black, has a pointed muzzle, larger ears and a scaly tail. Both these species can swim and can take an expert eye to differentiate.
For more information on the difference between water voles and brown rats see the The Wildlife Trusts website.
You should record the presence of protected species on a national database. This can done by contacting the local Statutory Nature Conservation Organisation (SNCO). Alternatively you can record online at the National Biodiversity Network. You should also alert the appropriate land owner, council and or management body to the presence and location of this species.