One Seat on the Rowboat

A zookeeper transporting animals reaches a river with a hawk, a snake, and a frog. The rowboat at the bank holds only her plus one passenger per trip. Left unsupervised together, the hawk will attack the snake, and the snake will swallow the frog; each is safe whenever the keeper is present, and the hawk has no interest in the frog. How does she ferry all three across intact?