First step: Break out the map and look for bridges or other crossings. If you recall, Chris McCandless got stopped by the Teklanika River 1/4 mile from a tram across the river. Hike to crossing and walk back down other side.

Second step: If that doesn't work, hike upstream and look for less lethal places to cross. Next, go downstream and repeat.

Third step: Accept that you're not going to cross the river today and either make camp for the night, or head toward toward a road that doesn't require crossing the river. EDIT: Now would be a really good time to break out the cell phone

Save the "heroic" solutions for TV personalities. You don't get paid enough to get yourself killed off camera.