Dams come in many sizes and shapes, everything from huge power generating hydroelectric dams to small low-head dams (called drowning machines) typically built to back up water for irrigation purposes. Near a dam, swirling water, submerged objects, and strong currents can pose dangers for boaters and swimmers.
Sudden discharges of water from spillways and turbines can rapidly increase river flows. Never fish or swim above or below a dam. To protect yourself when recreating on or along a river near a hydroelectric facility, follow these safety tips:
Low-head dams are generally small structures usually no more than 10 feet high. They have no gates or water control devices; water flows constantly over them. Because of their small size, they do not appear to be dangerous, especially when viewed from a boat or canoe upstream.
In the spring and during other periods of high runoff, however, the dams become very dangerous. Torrents of water pouring over the dam create a churning backwash or current. This "hydraulic," as it is often called, is really a recirculating current. The roiling water takes any object-including a person-to the bottom of the stream, releases it to the surface, sucks it back to the face of the dam, and pushes it back to the bottom. This cycle can continue indefinitely.
Branches and other debris trapped in the hydraulic pose an additional hazard to the victim. The victim has a hard time staying afloat, even if wearing a life jacket (PFD). Air bubbles mixing in the water decrease its buoyancy by one-third. These conditions create a nearly perfect drowning machine.
Watch the low-head dam simulation that shows how low-head dams are deadly traps of moving water. [Watch simulation ]
The Dam Safety Program regulates the construction, operation and maintenance of Montana's dams to protect life and property from damages due to failure. The dam safety program also provides training and outreach to dam owners and engineers, and assists with emergency preparedness activities.