A wave that strongly recirculates water upstream. The effect is one of 'keeping' whatever falls in it -- stick, picnic table, swimmer, boat.

Depending on the size of the keeper it may release large objects readily, but it's hard to tell until you're in it. Keepers are dangerous. Because of this they are sometimes called drowning machines. They occur most frequently below low head dams. They can be created by any sharp drop or large hole. If in doubt, stay out.