Funny that I just noticed this thread after yesterday's trip down a stretch of the local river. There's a spot just below the put-in that always has trees falling in or fallen in, often blocking the channel. Yesterday was one of those "blocking the channel" days. With yesterday's low current, there was ample time to evaluate the situation without getting pushed into it. We opted to line our canoes through the narrow slot near the bank, since there was zero headroom and angular current. All other obstacles on the trip were just a matter of sideslipping.
I'm always amazed by how much the river changes from year to year, due to falling trees and realigned channels.
There is an inlet stream here that I vist every spring when I can, because I like to view the heron rookery up close. The approach is often a maze of fallen and falling cottonwoods. One thing I watch for (because I'm slightly paranoid) and wasn't mentioned in Marc's article is the potential widowmakers. While it's unlikely that I'd be in just the wrong place and get hit as a tree falls, the odds of having a tree fall behind me and blocking my downstream exit are higher. Portage routes are very limited by thick vegetation, so every leaner demands evaluation.