River obstacles are serious hazards on any waterway - from placid, slow-moving streams to raging, multi-class rivers. While avoidance is most often the best safety move, knowing how to respond to impact/collision with an obstacle is critical to a paddler’s safety.
Physically backing away is a natural human response to avoiding dangerous encounters. Upon hitting a rock in a stream, for example, we might tend to lean away from the obstacle, out of harm’s way. As the situation worsens, and your watercraft is forced against the rock sideways by the volume/force of the water, you may try to lean even further away from the danger.
By leaning upstream, you are tipping the boat into faster, more powerful current dynamics that will increase the roll of the boat further into the lean - until the gunwales hit the water and the boat fills/flips completely.
Leaning downstream, towards the obstacle, allows the force to flow under the boat, lifting it and enabling you to shift weight and wrestle the boat free.
Trees/branches hanging down onto the water surface are called sweepers; those below the surface are strainers - very dangerous as the force of the current can literally sweep you out or over, or after capsize, pin you underwater against those branches. Upon capsize, one alternative to being entrapped within or under a sweeper (a large limb or debris pile, for example) is to swim/climb up onto it. This is VERY dangerous but may be your only alternative.
The most deadly obstacles are drops, either from ledges, spillways or waterfalls. If gravity doesn’t get you, the hydraulics or turbulent, circular flow of water at the base will! Natural and man-made spillway drops should be avoided at all costs. Their hydraulic action is nearly impossible to escape.
Other river obstacles include stump fields and submerged structures such as wiers and wing dams.