I agree with Ken V, and would also include those first trips into the Stikine by Lesser/McDougall/Ammons/Wasson & Co with those key events Ken mentioned that helped boost the transformation of whitewater boating into hyperdrive. I'll cite Doug Ammon's write-up
as Exhibit A for justification - great reading! I remember watching that American Sportsman episode with Lesser in the chopper flying down the Stikine canyon. He looked like he was about to wet himself at one point.
Powell's exploration of the Colorado was then - and remains now - an amazing chronicle. It stands on its own in the grander context of exploration, versus in the more narrow context of whitewater boating. It certainly has inspired and will continue to inspire whitewater boaters, but IMHO, I don't think it has been as transformational to whitewater boating as the introduction of rotomolded boats and some of these early, big, crazy runs in them.