Sure enough, a few hundred feet on from yesterday's photo, there were rocks! I suspect we were pretty lucky in our timing here; this was quite possibly impassable a few hours earlier. While "fall" in terms of seasons and dendrology comes between summer and winter, in terms of rocks, it can come at any time of year. However, it's most likely to come after prolonged and heavy rain, which often means mid-winter to early spring in this part of the world. This was not a spot for a leisurely pull-out and poking around; the heavy equipment was a little unnerving as we drove by. So I can't say with any certainty exactly what caused this, but that won't stop me from speculating over the next few days. I'd be surprised if the fact it was early March was completely irrelevant.
Speaking of rock falls, Twitter and IRL geofriend, Hollie (@GeoHols), was up the Sandy River Canyon on her way to the Columbia Gorge a few days ago with a geology class on a trip from Portland Community College, and saw a rock fall first hand. This is a great thing to have planned for a group of beginning geology students! ;-)
Photo unmodified. March 9, 2012. FlashEarth location (Fairly confident about this one, but not certain).