Not aware of any specific bridge plans. Go to Wick's site at http://www.zyworld.com/TheWick/ and look for some deck plans. Think of a deck with railings 30' long and 3' wide which is no big problem. Your problem is to span 30' though. You will need to get some railroad ties/poles and pile drive them in the creek banks or in the creek to shorten up the span. Somewhere on the net is a site that has lumber span tables and you can also look in some deck books. 2 supports evenly spaced will get you 10' spans which can easily be done with single 2by joists and the end spans can be sloped. You would probably end up with steel beams for a straight span of 30' but still doable.
You can go to your local home cheapo or lowest and they both have a computer program. Tell them what size and spans that will work and their computer will give you a drawing and parts list.