The beds of nonnavigable streams belong to the owner of the adjacent land. If the stream is a dividing line between two parcels of land, each owner's boundary shall extend to the thread or the center of the main current of the water. If the current changes gradually, the boundary line follows the current. If from any cause the stream takes a new channel, the original line, if identifiable, remains the boundary. Gradual accretions of land on either side accrue to the owner of that side.