Low-head dams create dangerous recirculation currents at the base of the dam known as a hydraulic. As water flows over the dam, a depression is formed. Water downstream rushes back towards the dam face to fill in this depression. This recirculation of water is often called a “drowning machine.” The features of a hydraulic include the following.
- Backwash: Water flowing back toward the dam face. Water in the backwash can be highly aerated. Aerated water may not support a person at the surface.
- Boil Line: Distinct feature that separates the backwash from the outwash. Water in this area appears to bubble like water boiling in a pot. Boats that cross the boil line will have a significant reduction in performance with both propeller and jet drive units.
- Outwash: Water below the boil line flowing downstream.