Thanks for the very helpful replies. After reading them, looking at some of the posted photos, and thinking it through again, I think I may reconsider my earlier statement that a bridge is out of the question. Anything I put down in the stream is likely to cause some disruption to the stream and may very well wash away during the heavy flow periods. Seeing the pictures, I also keep seeing the image of my 2wd tractor stuck smack in the middle of the stream -- possibly for days, with bad weather and rising water on the way...
Since I only need a carrying capacity of 10,000 lbs. (tractor, loader with full load, bush hog, operator), I think I can use a pair of 14"-34lb/ft I-beams at 35 feet long to span the entire creek with footings about 8 - 10 feet back from the edge (allowing for future bank washaway. A local steel supplier quoted me around $1,500 for the pair, including priming and holes punched for attaching wooden decking. Another $1,000 for railroad ties, lag screws, concrete, miscellaneous items, and free labor (me) might let me finish for around $2,500.
Will I still need permits, government approval, etc.? Any other counsel? Thanks.
Tom