Hole (hydraulic)
Also: hydraulic, hole, stopper, recirculation
A hole is a wave that flows back upstream behind a drop or weir. Strong holes can hold boats and swimmers.
- 1Drop or weir crest
- 2Hole with recirculation
- 3Surface water flows back to the drop
- 4Outflow downstream along the bottom
Wikibooks describes it as a counter-wave that breaks backwards across or diagonally to the current. Small holes can be run with momentum and a straight line, and you can even surf small ones. Uniform, wide holes below weirs, however, are life-threatening because there's no way out at the side.
Warning: never run a weir if you don't know what's below it. Uniform weir holes in particular look harmless and aren't.
Sources and further reading
Research as of 13 Nov 2026. We have marked manufacturer information as such.