The behavior described is typical of swans, both in good and bad
weather. I have observed it often during migration as the swans will actually
be flying down one river, come to an intersection with another and take that
river way as it heads south.
Particularly in bad weather the swans will stay low and follow rivers as
they seem to know the routes from this aspect. It is also safer since not many
aerial obstacles occur across the rivers. However, swans have been found dead
or injured after crashing into bridges.

Martha Jordan