How To Build A Bridge Over A Stream In A Forest With Only Trees, Bamboo ,rope ,chainsaw, Axe , And The Stream Current Is Strong , It Has 500 Crocodile In There. At The End Of The Stream Is A Water Fall?

Get all the stuff together and starting building the bridge on your side (where there's land and your safe from all the dangers). Seeing as bamboo isn't heavy, you'll be able to move the bridge and cast it over the stream to the other side, easily. If you need to know how to build a bridge properly I suggest the following:
1. Cut the bamboo into appropriate measure (wide enough for you to walk on, but not enough so its too heavy, long enough to reach from your side of the stream to the other side of the stream) for flooring to walk on.
2. Tie the flooring bamboo flooring securely, as so you can walk on it.
3. Put the flooring on more bamboo and test if the flooring can take your weight.
4. If it is secure enough continue, if not repeat step's 1-3 and then stack the flooring on-top of the other to make stronger and re-test.
5. Finally if you think you can balance the way over than throw the bamboo flooring over to the other side of your end to the other side of the stream and balance/walk across.
6. If you cannot balance/think you can't. Than simply make a small bar running across the bamboo and fix the ends of that bar to the land on your side of the stream as so that it doesn't move a lot.

Hope this helps, if not well than just notify me ill try to do my best.