I was watching a show on the National Geographic Channel that said beluga whales often swim in fresh water in Alaska. They do not live there, but they will swim in Fresh water streams and lakes to feed (on migrating salmon I think).
The show was about a lake monster report in an Alaskan lake. The Beluga whales were a possible suspect, but they were ruled out since they had to surface for air, meaning that sightings would be more frequent. In the end, the investigation concluded that the river monster was a White Sturgeon; a bottom-feeder that can grow to twenty feet long and live about a hundred years.