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 Maryland. 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.

Boaters getting swept into strainers is pretty common. Unfortunately, many have died after getting stuck on one of these obstacles. Do whatever you can to avoid strainers by ramming your boat to shore well upstream, get out and portage.