One of the joys of having a brook running at the back of my house.... (Next time, I'll have a better camera to hand!)

From the Cambridge Green Belt Project:
"The water vole (also known as the water rat) was made famous in Kenneth Grahame's 'The Wind in The Willows' and is the lovable character of 'Ratty'.
There are three native voles in Britain -- the field vole, bank vole and water vole. The water vole, however, is Britain's only aquatic vole, and its largest:
measuring about 20 cm in length (head and body) with a tail up to two-thirds its body length. They are relatively easy to observe,
and are active during the day. If disturbed, they will enter the water with a characteristic 'plop'.
The water vole has a wide distribution from Britain throughout Europe to East Siberia.

"National water vole surveys conducted by the Vincent Wildlife Trust have recorded a massive decline in water voles since 1989.
A survey in 1989 and 1990 found that water voles had disappeared from 67% of historical sites, and in 1996-1998 this had increased to 89%."