A logjam is just what it sounds like: a bunch of logs all jammed together, blocking the flow of water in a river or stream. You can also call other kinds of blockages logjams — like a traffic logjam that develops due to rubbernecking drivers trying to check out a crash.
Logjams change the way water flows in a river, sometimes creating new channels and pools from the overflow of water. This can be good for spawning fish, but sometimes a logjam causes flooding and other problems. This has given rise to the figurative logjam, a problem that stops or blocks progress: "The disagreement caused a logjam in Congress until the President stepped in."