"The Lady of Shalott" is stuffed with references to the natural world. Tennyson loops back again and again to the fields and trees and flowers that surround the island of Shalott. In fact, you might get a little sick of hearing about it. Still, the movements of nature (especially the endless flowing of the river) are a big part of this poem's rhythm; they help it all hang together.
Finally, it is not just Lancelot that the Lady wants to be in contact with, but the flowers and the river and the leaves. She is a prisoner kept apart from nature as well as from her fellow humans.
The story of Lancelot and the Lady of Shalott is inseparable from the natural world. The river and its surroundings are a crucial focus of the entire poem and they give it its structure and rhythm.