I found this explanation at BBC News:

Her five children are named Track, Bristol, Willow, Piper and, last but not least, Trig Paxson Van Palin. According to the Washington Post newspaper, Track was named after the course of the sockeye salmon the family fishes off the town of Dillingham, while her eldest daughter's name comes from Bristol Bay, an area known for its salmon fisheries. The name Willow relates to the state bird, the willow ptarmigan, and a nearby town, the paper says, while daughter Piper shares her name with the family's small plane. Trig is the Norse word for "brave victory", the Post adds.