Drowning Machines

by Richard Munn|Published 07-21-2006

Low head dams have resulted in so many close calls and fatalities that they have earned themselves the name "drowning machine."

What Are They?

low head dam warning signLow head dams are control structures used on rivers. As with all dams, they partially block the waterway and "back up" the water behind them, helping maintain water levels.

Simple in design, they are self-regulating and allow water to just spill over the top as water levels increase.

What Are the Dangers?

Low head dams are often deceptively gentle looking. The water seems to gently spill over the crest of the dam, move through a series of riffles and small waves, then continue downstream. The temptation to swimmers and boaters is almost irresistable - these structures look quite benign as they are approached from the upstream side.

The unseen danger is the strong current or hydraulic just beyond the downstream face of the dam. The water flow in this area is a recirculating flow, which drops down sharply down the face of the dam, rises, then returns to the face of the dam as a backwash where it quickly drops down again.

low head dam

The pattern is circular, and anyone caught in that recirculating flow will find it difficult if not impossible to escape. Boats, people and debris caught in that strong current could potentially roll around for hours before being flushed out.

Even a PFD will not provide sufficient flotation to prevent the user from being drawn underwater by this strong current.

Low head dams do not have to be large to create a dangerous hydraulic. Even a small dam can have a powerful backwash current, and if water levels are high (spring runoff or after a heavy rain) nearly all low head dams become increasingly lethal.

The danger exists not only approaching from the upstream end. Paddling up to the downstream side of the dam can be equally dangerous. As soon as your boat enters the turbulent area downstream of the dam, the current will pull the boat toward the dam where it will be pushed down the face. Anyone who has seen a boat caught downstream of one of these structures will vouch for the fact that it just sits there, rolling and rolling in the backwash.