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These are some of many river features you may encounter within moving water.


An upstream current that forms behind a surface object. It can be used as refuge from the main current.

Hazard: The “eddy line” between the upstream and downstream flows may be unpredicatble or have a dramatic effect on your paddle craft.

Low-Head Dam

This dam extends the entire width of the river and creates a hydraulic of recirculating water that spans the river, and a boil line which marks the separation of the downstream current (outwash) and the upstream current (backwash).

Hazard: Sometimes referred to as a “drowning machine,” this feature will trap and hold a buoyant object (such as a person or a paddle craft) in the recirculating water, continually forcing it back into the dam.

Debris is also often held by the hydraulic, creating more hazards for a person.

Escape is nearly impossible; rescue is extremely difficult and dangerous. The water in the hydraulic and just below the boil line is highly aerated, so paddle craft cannot get a good “bite” to make it through. The low-head dam is often very difficult to see from the upstream.


An obstruction that water is able to move through, but not solid objects. Examples are downed trees, gratings on culverts or pipes, and fences in flooded areas.

Hazard: The force of the current can hold a person immobile against the strainer with tremendous force and may be impossible to escape from.