In most cases, a sieve is a kayaker's worst fear.
As it turned out for the kayaker in this video, this sieve almost cost him his life. What you are about to see is as real as it gets. The video can be hard to watch, so use some caution. However, the kayaker makes it out alright--barely.
In case you're wondering, a sieve is a natural fast water stream occurrence during which water travels underneath a rock. As a result, kayaks can be bent and broken while caught in this fast moving water. Tragically, the kayaker can then continue to be forced under. As you just saw, considering all that happened, this kayaker is very fortunate to have been able to swim away to kayak another day.
Many people tend to think that kayaking is a nice recreational activity. However, since 2006, there have been 1,729 kayaking fatalities. There is little information about how these incidents occurred, but with hazards like you just saw, it's surprising there aren't more.