Roller dam

The world's first roller dam (German: walzenwehr) was constructed in Schweinfurt, Germany in 1902 to divert irrigation water south of the Main river.

Active roller dams are made from large metal cylinders, which can be lifted out of the water using a system of powerful hydraulic rams or cables and motors.

Any person going over the top of the dam will be caught in the rolling action at its base and may not be ejected from the cycle for days or possibly weeks.

Even very buoyant objects, such as inflatable balls, inner tubes, and life vests, can often be seen resurfacing near the downstream face every few seconds for several hours before escaping the so-called "washing machine of death".

Sixteen people have died by drowning at the roller dam on the Fox River near Yorkville, Illinois, since its construction in 1960.