Dams are built to back up water in a reservoir for a variety of reasons. Dams are hazardous both above and below the dam. These wall-like structures pool the water as it flows over the crest and drops to the lower level.
This drop creates a hydraulic, which is a backwash that traps and re-circulates anything that floats. Boats and people have been caught in this backwash. A person caught in the backwash of a low-head dam will be carried to the face of the dam, where the water pouring over it will wash him down under to a point downstream called the boil. The boil is that position where the water from below surfaces and moves either downstream or back toward the dam. A person who is caught in a low-head dam struggles to the surface, where the backwash once again carries him to the face of the dam, thus continuing the cycle.
To complicate matters, these dams are usually loaded with debris, such as tires and logs on the surface and rocks and steel bars just below, posing additional problems should a person get trapped in this dangerous structure.
Dams do not need to have a deep drop to create a dangerous backwash. During periods of high water and heavy rains, the backwash current problems get worse, and the reach of the backwash current is extended downstream.
Small low-head dams that may have provided a refreshing wading spot at low water can become a brutal death trap when river levels are up. Simply put, it is not the drop of the dam that is the lethal danger, but the backwash current. This backwash current is governed by volume of water and flow.
From downstream, you may not realize the danger until it’s too late. From upstream, low-head dams are difficult to detect. In most instances, a low-head dam does not look dangerous, yet can create a life-threatening situation. You should always pay attention to warning signs, markers or buoys and keep well clear of low-head dams.