Low-Head Dams: Why They Are So Dangerous

A low-head dam is a low wall built across a river, and water flows over it all the time. Below it, the water circles back toward the wall and can hold a person under, even when the drop is only a few feet, so stay well away from both the top and the bottom, and never go in to rescue someone: call 911 and throw a line from shore.

There are thousands of them on rivers and streams across the country. They often look harmless, which is part of the danger.

More than 1,000

What a low-head dam is

Colorado's Department of Natural Resources describes low-head dams as engineered structures built across stream and river channels (opens in a new tab), usually ranging from 1 to 15 feet high (opens in a new tab). Water flows over them continuously, from one bank to the other. They were built to divert water for farms, towns and industry, to control erosion, and sometimes for recreation.

The American Red Cross lists dams among the hazards to watch for (opens in a new tab) in rivers and lakes, and there are a lot of them. Researchers at Brigham Young University built the first national inventory and located more than 13,000 low-head dams (opens in a new tab) across the country, and they say there are more.

Size doesn't tell you how safe a dam is. Virginia's Department of Wildlife Resources (DWR) warns that the danger is as great with a two- or three-foot dam face and a moderate flow (opens in a new tab) as with a big drop.

The recirculating current below

Virginia DWR explains what happens at the base of the dam:

Water flowing over a drop forms a hole or hydraulic at the base which can trap objects washing over the drop. Backwash or recirculating current is formed below the dam. Once swept over the dam, a victim becomes trapped and is forced underwater, pushed away from the dam, then circulated to the top. (opens in a new tab)

The cycle repeats as the person is drawn back toward the dam. DWR adds:

• The concrete walls at the side of the dam face block the exit route (opens in a new tab) for people trying to escape.
• Areas immediately downstream also present risk, because the water is flowing upstream (opens in a new tab). That matters for anglers and waders below a dam, not only boaters.

The National Park Service gives the same warning for rivers in general: hydraulics, or recirculating currents, can hold a person underwater (opens in a new tab).

How to spot one from the water and the shore

From upstream, a low-head dam can be almost invisible. Colorado DNR notes that the calm pool behind the dam gives no sign of the danger just beyond the horizon (opens in a new tab). Virginia DWR adds that dams are difficult to spot from upstream and often are not marked by signs or buoys (opens in a new tab).

What to look for:

• A smooth horizon line where the stream meets the sky (opens in a new tab). Virginia DWR says it can mean a dam ahead.
• Concrete retaining walls (opens in a new tab) on the banks, which DWR notes are easier to spot than the dam itself.
• A white line of churning water straight across the river. BYU researchers used this tell-tale sign of whitewater on the downstream side (opens in a new tab) to find dams on satellite images.

Before a trip:

• Scout the river. DWR advises: scout the river and know the location of hazards, and talk with boaters who know the river (opens in a new tab). Colorado publishes a map of known low-head dams (opens in a new tab), and the BYU team's national inventory (opens in a new tab) is online too.
• Plan to carry around. Get out well above the dam and carry your boat or tube on land. DWR says to re-enter the river well downstream of the boil (opens in a new tab).
• Wear a life jacket. The National Park Service's first rule of river safety is to always wear your PFD (opens in a new tab), or life jacket.

If you are caught

Virginia DWR is honest about this: it is nearly impossible to escape the strength of the hydraulic when trapped (opens in a new tab). Its advice is to tuck the chin down, draw the knees up to the chest and wrap your arms around them (opens in a new tab), in the hope that the current pushes you along the riverbed and past the boil line, where it lets go. The real lesson is to never get close.

Rescue: why bystanders must not go in

Watching someone struggle makes people want to go in. At a dam, that can lead to a second victim. Virginia DWR says rescuing trapped individuals is dangerous and often unsuccessful (opens in a new tab), and that untrained rescuers should never approach the hydraulic in a boat (opens in a new tab). The BYU research found that about 25% of deaths happen when people try to rescue someone and underestimate the current (opens in a new tab).

Colorado DNR notes that someone caught below a dam often requires prompt, qualified rescue (opens in a new tab). Your job is to get those rescuers there fast: call 911, give the exact location, and keep the person in sight. Learn the rule in reach or throw, don't go.

What you can do today

• Look up whether there are low-head dams on the rivers and creeks near you, and where they are.
• If you paddle, mark every dam on your route and plan where you'll get out and carry around.
• Teach your family to recognize a smooth horizon line and concrete side walls as warning signs.
• Agree as a family: if anyone is caught at a dam, we call 911 and throw. Nobody goes in.

For more on moving water, read river safety and kayak and canoe safety. If you fish below dams, see fishing safety.