The western water shrew ranges from southern Alaska to the southwestern United States. It is semiaquatic, swims and dives well, and eats aquatic insects.
Total length is about 14–16 cm, including a 6–8 cm tail. Its glossy blackish-gray fur is paler or silvery underneath. It has a long bicolored tail, large hind feet fringed with short stiff hairs, and partly webbed toes.
It uses streamside conifer forests, often beneath overhanging banks or in crevices with good cover. It is also found at seasonal streams and small seeps, where it swims and dives for aquatic insects.
An extinction-risk assessment has not been added yet.