How about this:
At the waters edge, by lashing, build a semi-rigid "pole" of roughly harvested trees longer than the river is wide. Use as few trees as you can to minimize the number of lashings. "Hinge" the pole solidly at the downstream end with whatever you have or make. Make yourself as buoyant as possible to insulate your core as much as you can. Push off on the upstream end of the pole and the current will rapidly "close the door" by pushing the upstream end of the pole across the river - where you better get off pretty quickly before the river destroys the "door."
What TV show did you get this ill conceived idea from?
it seems the fellows that died in zion were would-be contestants:
Zion National Park (UT)
Two Rafters Drown In Virgin River
On Sunday, April 25th, rangers received a report of an overdue party from a float trip down though the Virgin River Narrows. Investigation revealed that two 23-year-old men from Las Vegas had hiked into the Narrows with the intent of constructing a log raft and floating approximately 50 miles through the Narrows to Hurricane, Utah. The men were not equipped with wetsuits or PFDs, did not have whitewater rafting experience, and had limited camping experience, little food and no overnight gear.
They told their father that they intended to record their entire trip on video camera as an entry into the “Man vs. Wild” competition. The SAR operation concluded on April 26th when both bodies and a small amount of personal gear were recovered from the river.
ps: At the time, the north fork of the Virgin River was running about 250 cubic feet per second with water temperatures of 40 degrees.