Whitewater
Strainers, Sieves And Undercut Rock
Obstacles that let water through but not a body are the most lethal river hazards, because the current continues pushing while the person is held against the obstruction.
Most river features release what enters them. A small category does not, and it accounts for a disproportionate share of whitewater fatalities.
The defining property
A strainer is anything that filters water through gaps too small for a person, typically a fallen tree with branches in the current.
Water passes freely, so there is no cushion pushing objects away. A body arriving is pinned against the obstruction by the full force of the current behind it.
That force does not tire. This is the reason these features kill people who are strong swimmers in water that is not especially fast.
Sieves and undercuts do the same in rock
A sieve is a jumble of boulders through which water flows in gaps a person cannot pass, and it behaves exactly like a wooden strainer.
An undercut is rock the current has eroded beneath, so water flows under a ledge. A swimmer carried into it is pushed below the rock rather than deflected off it.
Both are hard to see, since the surface downstream of an undercut can look ordinary, and the absence of a pillow of water piling against the rock is the giveaway.
Why the response is counterintuitive
The defensive swimming position taught for river swims, feet downstream and on the surface, is exactly wrong for a strainer, because it feeds the legs into the obstruction first.
The taught response is instead to swim aggressively toward the obstruction and attempt to get up and over it before the body is drawn beneath.
That is a trained reaction requiring practice in controlled conditions, and it is one of the core reasons swiftwater rescue courses exist.
Wood changes after every flood
Unlike rock features, strainers appear and disappear. A tree that fell last week is not in any guidebook and was not there on the last descent.
High water moves wood downstream and deposits it at constrictions, on the outside of bends and at the head of islands, which is where paddlers look for it.
This is why current information from people who have run the river recently is more valuable than a description written years ago.
Avoidance is the whole strategy
Because rescue from a strainer is extremely difficult and often impossible, everything sensible about them happens before contact.
Scouting blind bends, keeping to the inside of corners where current is slower, and portaging where wood blocks a channel are the standard measures.
Learning to identify these features from a boat, and the rescue techniques used when someone is pinned, requires formal instruction and practice with qualified providers.
