I would consider renting or borrowing (this is better if you know a friend who has some) four 20ft long aluminum walkboards that are 12" wide (or three 16" wide), like the ones used on scaffolding when building/painting houses, etc. I know you said the creek was 15ft wide, but going 20ft allows 2 1/2 ft overlap on each end.
I think if three or four of these walkboards were laid side by side, that it would provide a solid frame for a four foot wide bridge. I would then come up with a temporary way to anchor each end of the walkboards to something solid driven into the ground. After they are anchored, lay some 4x8 sheets of 9/16" thick plywood sheathing on top to cover them (could use 3/4" if you wanted a little more strength).
Since you can't really attach the plywood sheathing to the aluminum walkboards themselves, I would use a 20ft long 2x6 on each side and use that to tie the plywood sheathing in place. THen, come up with a railing on each side, probably dig some holes and plant some 4x4 posts in the ground (that are also tied in with the 20ft long 2x6's on each side), and use some 20ft long 2x4's for hand/side railing spaced a foot apart. All this "text" may not make too much sense, but I can picture it in mind a lot easier than I can 'splain it here lol!!!
I think a bridge like that could be constructed in a day and a half at the most, once you had all the material on site. A 20ft walkboard can be drug or carried in by two people fairly easy. The rest of the materials would be pretty light and could be carried by two people making a number of trips. At 4 foot wide, it would be wide enough for anyone in a wheelchair to cross it too. I would certainly limit the number of people crossing the bridge to no more than two at a time, just for the sake of being safe.
Something to think about anyway. Good luck with it!