Something of that span would probably have to be engineered.What sort of load would it carry?- what material are you planning on using to construct it?I built a small footpath bridge over a winter creek-I just built a form and poured concrete over reinforcing rod.20' is quite a span if you are using stone or concrete, and something like that would require engineering, I'm sure, simply because of the weight alone.I think in the old days, they cut stone to fit.
An arched bridge is much more difficult to build than a regular bridge. Something that would be fairly simple, and very slightly arched: get an old, big, flatbed trailer. Take off everything that doesn't look like a bridge. Attach to concrete piers at each corner.
This site has information on building bridges on site, but I don't know if you want a 'foot traffic' or vehicular traffic bridge. It is a International Peace Corp type of organization, who service clients in extreme primative conditions - maybe you can find your information here. I own their VITA handbook - it is great.
SteveD's idea sounds very good, if the road to your site is trailer friendly all the way. Shouldn't be much trouble for a big rig to drive it there, unhook and split. If you are worried about having to drop bridge, why not ramp both ends up to it and go from there.
Good luck with your bridge. Also do a search on this site - I seem to remember within the past year, someone asking about a suspension bridge for foot traffic and someone had a post with a great site for it.
Going to church doesn't make you a Christian any more than standing in a garage makes you a car.
How about a rope or cable bridge? You could still cover it with wood... and it may be a whole lot easier to build and move.
I doubt I'd go with 30' glu-lam arched beams... 30' straight steel beams which spanned the stream... and just "attach" your wood arched bridge to them. Obviously, the steel beams do not have to be visable.
Cheers from Southern Ontario CANADA!