Thanks for all the suggestions.

I think I'm going to try the straight across route. It's hard to
measure the actual distance from bank to bank, because the banks are not
straight lines and they slope down to the creek (not a sheer drop). It
could even be as short as 10' in some places - but I'm trying to be
conservative in my estimates so I don't end up with a bridge that's too
short. The actual width of water in the creek is typically about two to
four feet, depending on how much water is flowing. If it's been dry,
there will be just a trickle or no water in it at all for much of the
summer.

I won't be pouring any concrete or digging down to the frostline. I'm
planning to just rest the bridge on top of a couple of 4x4s at each end
- maybe dug half-way into the ground.

Thanks.

-Jonathan