I think Yatzek has a good idea. Piers or posts may be problematic if there creek floods or the ground 'moves' a lot at the creek's edge. Usually, piers are poured onto bedrock and anchored there. I don't think you want to get into that unless bedrock is relatively shallow.
With a suspension bridge, you can pour piers further away from the creek's edge and most likely not worry about 'movement' in the ground so much. If you go this route, plan ahead for the capability to 'tighten up' the bridge because, well, I'd assume that 'things' are gonna move on ya. Perhaps you could contact a Trade School or the Engineering Dept. at a local college and see if a student wants to do this for a project. This is a good time - Some schools haven't started their Spring Semester yet!