One of the major causes of migration for mature beluga whales is breeding. They need to meet for reproduction and move toward warmer, shallow waters during this time. For the majority of belugas this means a long migration south, but depending on the environment some groups will stay by these warmer coastal waters all year long and not need to migrate at all for breeding.
The summer months mark the regular migration patterns of the beluga whales as mothers move to shallow waters for calving. The summer months are the most likely time to see large populations of belugas where the rivers meet the ocean. Some of these groups of belugas will even be visible in the estuaries of rivers during calving time.
The migratory patterns of different pods of beluga whales can vary greatly. Some groups never migrate very far, always staying in one area. These pods are usually in coastal areas in the southernmost sections of the beluga's natural habitat. On the other hand, scientists have found at least one pod which travels from the Northwest Territories of Canada in summer all the way to the southwest section of the Bering Sea in winter, which covers over 3,000 miles.
Environmental influences can strongly factor into the migration. The pods of beluga whales furthest north in the summer are also most likely to travel the farthest in winter, while whales near coastal areas in subarctic waters might rarely migrate all that far at all. Given a big enough river in arctic or subarctic areas, a small pod of belugas may even travel upriver. The Yukon River of Alaska, St. Lawrence River of Canada and Amur River of Russia all have recorded instances of belugas swimming hundreds of miles upstream.