Snags

And, to add insult to injury, randomly, wood which has fallen into the stream (branches, logs, and whole trees) will occasionally accumulate at random points.

Again, all these things will change year to year, month to month, even day to day! Every time flows go up to runnable levels, new areas of concern will arise, which is why (on something this narrow and continuous) it is virtually MANDATORY to walk the whole section of creek before putting on!