These are monarch butterflies. Monarchs are unable to survive cold winters, so they migrate during the fall to southern Mexico. In forests there millions of monarchs will spend the winter before flying back north. Their migration creates a river through the air. It’s a beautiful sight to see and if you’ve never seen it just spend some time on the quad when it is just beginning to get cold. Monarchs have already had several close calls with extinction, so make sure to see them before they’re gone.
I like the way you form a metaphor. Through comparing the migration of monarch butterflies to a river, a vivid image is clearly created.