We are familiar with strainers – those downed trees that stick out into the water, that catch and upend boats, while letting the water flow through them. There are two other water hazards of flowing water that are deadly: Hydraulics below dams, and siphons where water flows below a large obstruction and can suck an unwary or unlucky paddler deep below the surface and keep them there.

Water flow below a dam often creates a large area of foam as the flow over the dam hits the pool of water below the dam. The flowing water actually then proceeds to flow below the level of the pool, propelled by momentum and gravity, and creates the foam and a backward moving reverse curl, sort of a vertical eddy.

Boats and unlucky paddlers get caught there, and can’t get out.  For one thing, bubbles do not support boats or people, so the boat or person sinks below the level of the surrounding pool. And then, the reverse curl pushes them back upstream into the incoming flow from off the dam.

I think the effect of the bubbles and foam created below the dam is underestimated by most of the experts and diagrams. You can’t hope to float enough to get your breath if you are caught in the foam. Paddles and arms don’t get enough purchase in the bubbles to propel you out of the hydraulic, and boats won’t float out of the hydraulic either.

A siphon is formed when water flows UNDER a large obstruction, such as a boulder or section of a concrete dam that has been undermined.

For years, there was such an undermined dam just upstream from Palmetto State Park on the San Marcos River, called Ottine dam. There was an obvious whirlpool upstream from the dam.

Floating objects were seen to be sucked down into the whirlpool, and never resurfaced downstream.

 

 

This is the siphon where water flowed UNDER Ottine dam. The San Marcos River just disappears under the dam. Ominously, the pool below the dam was still water, and the flow and any boils were not evident until much further down river.

Also, the remains of Old Mill dam that used to be near the San Marcos River Retreat used to have a section of concrete off to river right that would suck you under if you didn’t make the turn to the left quickly enough.

The danger is, of course, that once you get sucked under, you might not fit through whatever hole the water is going through, or get snagged on whatever rebar or other junk is down in the hole, and not be able to get back to the surface.

A member of Alabama Etowah County Rescue Squad drowned while attempting to search for a drowned kayaker at this Low-Head Dam in Big Wills Creek in Gadsden, Alabama. April, 2015.

The best advice is to AVOID these at all costs. Portage around both Siphons, and Low-Head Dams. It seems like every year we get reports of boaters drowning in the hydraulics below low-head dams. Worse yet, practically every year there are reports of RESCUERS getting caught and drowning in the hydraulic while attempting to rescue the original boater, or search for a drowning victim.

Stay safe out there! See you On The Water!

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The author, Harmon Everett