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In light of the recent deaths in Yosemite, I thought it appropriate to issue a warning about a very real danger in the rivers of our country that is often overlooked - weirs, low-head dams, and other examples of what have been dubbed "Drowning machines".
All low head dams are not created equal. Vertical rock dams permit the water to spill directly into the pool below, creating a short reverse eddy as the water falling from above forces its way to the bottom drawing downstream water with it. This "roller" or "boil" is usually short and occurs only within the first ten feet of the pool. Fishermen should never venture too near this boil for fear of being drawn into the vortex and drowning. Stay back from the depths of the pool for all your fishing.
Far more hazardous are the low head dams with a sloping spillway which often have a "stilling basin" of concrete that extends some distance from the face of the dam. These are highly dangerous structures that create multiple backwash currents. According to the State of Pennsylvania. "The most dangerous hazard on a river is a low-head dam. There are more than 2,000 such dams on rivers and streams throughout Pennsylvania, and they are true "drowning machines." Water going over a dam creates a back current, or undertow, that can pull a boat into the turbulence and capsize it. This hydraulic can often trap and hold a person or a boat. Dams do not have to have a deep drop to create a dangerous backwash. During periods of high water and heavy rain, backwash current problems often become worse, extending farther downstream. A small low-head dam that may have provided a refreshing wading spot at very low water can become a monstrous death trap when the water level rises."
Watch this video of trained professionals trying to recover the body of a firefighter killed the day before and still stuck in the boil - http://www.youtube.com/watch?v=mtR-mliLoP4&feature=related
Consider all low-head dams, weirs and such, as points of extreme danger, not places to play. Please.
Here is a list of links on low-head dams from a family that lost a son - http://www.charlesvolpe.org/ContactCity.html