In a river, they're called strainers. If someone is caught against one, water pressure often makes it impossible to extricate themselves without assistance, and if their head is underwater when they become trapped, rescue may not come quickly enough.

They're also less predictable than a lot of other river hazards. It's possible to come around the bend and be faced with a big strainer that wasn't there the last time you ran the river.

Here's a good video showing a rescue from one: https://www.reddit.com/r/whitewater/comments/cn6k8k/nasty_st...