Moss does like dampness but I'm not sure how it will fare when your creek is full. If it dries out then floods the moss may just float away. But I hope that works for you.
My first thought was Louisiana Iris, and I think they will do great for you there. They like wet feet, can grow in water or in soil and they bloom when they have been wet and then dry out. They also form very sturdy mats of rhizomes and roots that should help hold the soil along the sides of your creek bed.
