What is the purpose of a low-head dam?

Boat and PWC operators may encounter physical structures such as dams, locks, and bridges. You need to be extra cautious in these situations.

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Safety tips
What is a Low-Head Dam?
Why is a Low-Head Dam Dangerous?
Just How Dangerous is a Low-Head Dam?
How to Stay Safe

Dams pose dangers both above and below the dams.
The low-head dam is the most dangerous type of dam and has been named the “drowning machine.” They may not be easily spotted because the top of a low-head dam can be several feet below the water’s surface. Because of their small size and drop, low-head dams do not appear to be dangerous. However, water going over a low-head dam creates a strong recirculating current or backroller (sometimes referred to as the “boil”) at the base of the dam. Even on small rivers, the force of the backroller can trap your boat against the face of the dam and pull you under the water—even while wearing your personal flotation device (PFD). Be aware that on large rivers or during high water, the backroller or boil may be located more than 100 feet downstream of the dam. Avoid low-head dams.

Low-head dams pose a serious danger to vessel operators. Surface currents below low-head dams can suck vessels toward the face of the dam. Currents above low-head dams can sweep vessels over the dam. The recirculating currents and turbulent waters below these dams can swamp vessels and drown boaters.

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The low-head dam is the most dangerous type of dam and has been named the “drowning machine.”

They may not be easily spotted because the top of a low-head dam can be several feet below the water’s surface.

Because of their small size and drop, low-head dams do not appear to be dangerous.

However, low-head dams pose a serious danger to vessel operators, both above and below the dams.

Surface currents below low-head dams can suck vessels toward the face of the dam.

Currents above low-head dams can sweep vessels over the dam.

The recirculating currents and turbulent waters below can swamp vessels and drown boaters. Low-head dams vary in height from one to several feet below the water’s surface. Though their drop may be small, you must never assume you can go over without danger. And remember that water going over a low-head dam creates a large recirculating current or backroller (sometimes referred to as the “boil”) at the base of the dam.

Even on small rivers, the force of the backroller can trap your vessel against the face of the dam, and pull you under the water—even while wearing your personal flotation device (life jacket).

Be aware that on large rivers or during high water the backroller or boil may be located more than 100 feet downstream of the dam. Avoid low-head dams.

A low-head dam is a man-made obstruction typically built within a river or stream channel, and spanning from bank to bank.

Many variations exist among the thousands of low-head dams on streams and rivers throughout the United States, but they generally:

have water flowing across the entire length of the top of the dam,
are from 1 to about 15 feet tall (although taller ones exist on very large streams),
store a minimal amount of water below the stream bank level, within the channel, and
do not typically provide flood reduction storage.

Why are they dangerous?

Low-head dams can be deceivingly dangerous. At times the water around them appears tranquil and inviting; however, moderate to high flows over such dams create strong turbulence and recirculating currents that can push victims underwater, and then pull them back to the face of the dam in a repeating cycle.

These structures have been known to trap unsuspecting victims in dangerous conditions. Even the strongest swimmers and rescuers wearing the best personal floatation devices have drown at low-head dams.

Map of low-head dams in Indiana

Safety tips

Know hazard locations

Familiarize yourself with the stream. Know hazards along your route and plan to portage around them.
Low-head dams can be difficult to locate and are usually not marked.
When looking downstream, a smooth horizon line where the sky and stream meet or concrete/stone retaining walls may indicate a low-head dam.

Avoid low-head dams

Never attempt to paddle over a low-head dam. Don’t get too close.
Portage around the dam or paddle back to your put-in spot.
When portaging, go far enough upstream from the dam so you can safely make it to shore before being swept over.
To re-enter, go far enough downstream that you do not get swept back into the current of the dam.

What to do if trapped

Tuck your chin into your chest, draw your knees up and wrap your arms around them. Conditions may push you out of and away from the hydraulic current, along the streambed.
If a fellow paddler becomes trapped do NOT jump in after them. Do NOT approach the hydraulic from a boat in attempt to rescue.
Get to the shore and call for help. About 25% of all low-head dam deaths result from attempts to rescue another person.

More resources

Over, Under, Gone: The Killer in Our Rivers

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On a river there can be many dangers. Some are easier to pick out than others. One that many people overlook are low-head dams. In fact, ask the average person what a low-head dam is and they will have no idea.

Let’s take a look at what exactly a low-head dam is. Also how recirculating currents are created by them. And finally how you can see safely navigate a river near them.

What is a Low-Head Dam?

Low-head dams are also called weirs. Like any dam they are meant to alter the flow of water in a river. But unlike something like a large hydroelectric one, a low-head dam is much smaller. Often they look like concrete barriers that span a river from shoreline to shoreline. They are designed to raise the water level upstream in a river.

A low-head dam may exist for any number of reasons. They can assist in the generation of hydroelectricity. They can also assist navigation for boats. Despite the practical purposes of some low-head dams, there are also dangers.

Why is a Low-Head Dam Dangerous?

There are hundreds if not thousands of low-head dams across America. Many of these dams date back over 100 years. They were often designed to assist power generation in factories. These dams directed the flow of water to turn wheels, for instance. That in turn could allow a mill or other business to run.

As technology evolved, low-head dams became less necessary. While larger dams are maintained, low-head dams are often ignored. Because of their small size, they can easily fall into disrepair. A failing dam is not going to flood a valley or town like a hydropower dam or other types.

Unfortunately these are safety hazards. They cause what is called a subcritical flow upstream. That means before the dam the water is slow and deep along the face. After the drop on the back side it becomes supercritical. That means it greatly picks up speed.

When the fast current in rivers hits another slow current it can be dangerous. A recirculating current is formed. Something called a backwash or hydraulic jump is present. Water rushes over the top. But then other water rushes back to fill the depression it creates after the drop. That area is a safety hazard for swimmers or boaters. It is turbulent and quick moving. It rotates and can easily pull small boats or swimmers under if the current is fast enough. The backwash/hydraulic jump makes unpredictable currents. This is where most victims lose their lives.

Additionally, the upstream side presents a risk. The water passing it is accelerating. Depending on how the dam was formed, a person or boat could be pinned against the upstream face of the waterway. These days, with the number of damaged and failing dams, this risk is even more prevalent.

Low-head dams are often clogged as well. Debris, branches, and other objects can collect on the structure. If you get trapped near one, you and your boat can be damaged. Many times stronger, heavier debris will be trapped below the surface. This could include steel and rock.

Just How Dangerous is a Low-Head Dam?

These are serious hazard locations as environmental engineer Bruce Tschantz points out. He describes a low-head dam as a drowning machine in rivers and streams. They have caused hundreds of deaths. More than any other kind of dam.

Often a these can look innocent when water levels are low. In fact, they often make for appealing swimming and fishing spots. But when water levels rise, backwash current increases drastically.
A low head dam can often be hard to see from upstream. Downstream they often look calm and innocent. That is the reason they have proven to be so dangerous.
Because of the speed of the water present, it will typically be very cold. That can increase danger of hypothermia should you get trapped.
Low-head dams recirculate a lot of air. This churning air and water reduces buoyancy. That can make it even harder to stay afloat. Combine that with the speed of the backwash current. This can lead to drowning even if you are a strong swimmer.
These structures typically span an entire river. That eliminates the chance for any safe haven like you might find in natural rapids.

How to Stay Safe

Ideally you will see signage near one. They should be marked to help people avoid danger. If you are in a boat, like a kayak, it is best to portage around the dam. Navigation is too risky. Make sure you have a flotation device.

If the area is not clearly marked the structures may not be in a state of good repair. If it is neglected the risks could be many. And also unpredictable. If you discover one with no warning signs or rules, be wary. It’s best to avoid this area altogether just to be safe.

The flow conditions near a low-head dam are too unpredictable. What may be trapped below the surface will be wholly unknown. For those reasons, any time you swim, fish or boat near one you are at risk. Choose a spot further downstream or upstream to be safe.