According to this undated story from the ESPN Outdoors website (yes, that ESPN), the New Zealand mud snail (Potamopyrgus antipodarum) has been found in California. Previously seen in rivers in Idaho and Montana, this tiny snail is prolific and very difficult to kill. This is a decent, fact-filled, environmentally-conscious article, with pointers for fishermen on how to avoid spreading this and other invasive species.
Researchers are not exactly sure how the New Zealand mud snail was introduced, although likely vectors include ships' ballast and imports of gamefish (as eggs or adults). Work continues to measure potential effects of introduced mud snail populations, with initial studies suggesting they contribute to the decline of native aquatic invertebrates.