The Water Vole, Microtus richardsoni, is the largest North American vole. IT is found in the northwestern United States and southern parts of western Canada. THis animal has been historically considered a member of genus Arvicola, but molecular evidence demonstrates that it is more closely related to North American Microtus species.THese animals have gray-brown or red-brown fur with gray underparts. THeir large hind feet help make them excellent swimmers. THey are 25 centimetres (10 in) long, with an 8 centimetres (3 in) tail, and weigh about 110 grams (3.9 oz).