There is an overhanging tree, a sweeper, on the San Miguel River in Norwood Canyon. It blocks most of the river, and cannot be seen until you are almost on it. This is about 1.5 hours downstream of the Norwood Bridge, and a little above the campsite marked with a post with the number 1. It is right below a gravel island, where the right channel is fairly direct, and the left channel has a bend at the end. If you take the river right channel, you will be good, you will see the tree and dodge it easily. If you take the river left channel, you will not see the tree until you are almost on top of it. At that point, the current is fast, and the only way to save yourself is to aim for the island and stop. If you are in the middle of the left channel, pointed downstream, you will be in big trouble.