Strainers are a very common river hazard on small streams or creeks. Be very
careful on recently flooded rivers, especially those with muddy banks. As the
river cuts into these banks, trees get undercut and topple over. Just like an
iceberg, the real danger might not be obvious. Small limbs under the water let
water pass through but may impale a swimmer or may trap them. Strainers often
park themselves in constricted sections of the creek where the water current is
really powerful. Here is an example of a strainer on the Savage River in Western
Here is a close-up of a strainer on the Esopus Creek in New York: Esopus Creek Strainer. Trees are not the only types of strainers, pipes and low bridges that allow water to pass under them as also quite dangerous. Here is an example from the AW site: AW Pipe Strainer.
Early detection and avoidance are the best way to deal with strainers. Be very aggressive in steering clear of strainers. If you come across a strainer that can't be safely avoided, ram your boat ashore upstream if possible and get out. If swimming with your boat and it looks like you can't safely steer the boat away from the strainer, push off the boat and swim quickly away from the strainer. Strainers have claimed the lives of a number of boaters over the years. You can do a search on the AW Accident Database via this link: AW Accident DB Search. I know there are many more incidents than have been reported in this database.
Assuming you can't avoid the strainer, the typical defensive swimming approach - feet downstream with a back stroke will not work. Once your feet go under the strainer, the rest of your body will do the same. The best approach is to face downstream in an american crawl, swim aggressively towards the strainer, launch/push your body over the log and kick hard to get the bottom half over the strainer. Sometimes you can climb a strainer pile as there are a number of decent hand and foot holds. Do whatever you can to prevent any part of your body going under the strainer. In the AW SWR class, we practice this technique with an artificial PVC pipe strainer. Here is an excellent video demonstration of this drill: Strainer Drill.