LOW-HEAD DAM BASICS

A low-head dam is a smaller, human-made structure built in and across a waterway. Most are between 1 and 15 feet tall are designed so that water continuously flows over them. Unlike other dams, low-head dams do not tower above the water and typically do not include other parts to hold or release water, like gates or separate spillways used to pass water around or through the dam and downstream. Many were built to regulate water to operate mills, generate power, grow crops, supply water, or deepen waterways for recreation. While present throughout the U.S., many have outlived their initial purpose and original owners.

Dams listed in the National Inventory of Dams meet the following criteria:

• Dams where downstream flooding would likely result in loss of human life.
• Dams where downstream flooding would likely result in disruption of access to critical facilities, damage to public and private facilities, and require difficult mitigation efforts.
• Dams that meet minimum height and reservoir size requirements, even though they do not pose the same level of life or economic risk as those above – these dams are typically equal to or exceed 25 feet in height and exceed 15 acre-feet in storage, or equal to or exceeding 50 acre-feet storage and exceeding 6 feet in height.

Dams listed in the low-head dam database are typically:

• Built across a stream to pass flows from upstream over all, or nearly all, of the width of the top of the dam.
• Do not have a separate spillway or spillway gates used to pass water around or through the dam and downstream.
• Range from 1 to 15 feet in height.
• Built for a range of purposes (e.g., check dam, mill dam, irrigation, water supply, recreation, hydroelectric, or cooling pond), but in all cases, it provides little or no water storage function.

Some low-head dams can be harmful to the natural environment and make it difficult for aquatic life to thrive. Low-head dams may prevent fish from migrating to reach overwintering habitat, access different feeding zones, avoid predators, and spawn. Also, fish seek cleaner water when low oxygen or pollution reduces water quality. Over time, the foundation material dams are built on can erode and release trapped sediment, resulting in decreased water clarity and loss of habitat in addition to creating concerns with the structure itself.

Like any dam, a failure can result in floods, with water flowing freely downstream and lower water levels upstream. There are additional hazards on low-head dams that can pose dangers to the public. Read more about these hazards and actions you can take to be safe on the Association for State Dam Safety Officials website.

These dams are often “nonjurisdictional,” meaning that they are not part of any state or federal dam safety and inspection program. Often the ownership cannot be easily determined, and in many cases the low-head dam is either abandoned or no longer serving its purpose. Generally, the entity who originally built the dam has responsibility. People wishing to learn more about low-head dams including where they are and who is responsible for them can view available data on the Low-head Dam Inventory. More work is needed to improve available data - including the quality of what is publicly available and addition of information for low-head dams not yet captured on the site.

Dam owners and regulators regularly assess dams to understand and improve their safety and effectiveness. Removal of structures can be considered during these routine evaluations. Owners or regulators of low-head dams wishing to remove a low-head dam within their responsibility can consult resources available on the Association of State Dam Safety Officials website. In addition, some states have published guides and lessons learned removing or modifying these dams. It's important that people living and recreating near these dams are a part of the conversation and engaged in decisions related to low-head dams.

It's important to know the various words for identifying low-head dams depending on the context in which they're discussed. Low-head dams are also called low overflow structures, run-of-the-river dams, weirs, diversion dams, and grade control structures. When in doubt, ask an expert to understand if a dam near you is a low-head dam.

• Low-head dams are built across a waterway, often called run-of-river or overflow dams. Water flows over the dam continuously, moving from an upstream source downstream.
• They're usually low in height - 1 to 15 feet tall - making these dams hard to spot from afar.
• These dams create a drop-off that can excite and trap recreators on the water.
• Water flowing over a low-head dam creates a circular current at the base of the dam. This area is often called the boil zone that's characterized by water that appears to bubble or boil. Air bubbles mix into the water decreasing the buoyancy by one-third, which makes staying afloat even more difficult (even with a life jacket).
• As water flows over the top of low-head dams and falls, it creates a recirculating current that can pull people and debris down, up, and back toward the dam in an unrelenting cycle.
• Low head dams are usually made of concrete or masonry, which is slick and makes climbing out of the water immediately near the dam nearly impossible.
• When in doubt, get out and scout! Prepare and scout your route in advance to locate potential dangers, like low-head dams.
• Avoid going over or swimming near low-head dams.
• Contact an expert if you want to learn more about low-head dams near you and how to be safe in the waterway.

All known low-head dams are captured in the Low-head Dam Inventory. You can view dams near you by interacting with the available map. When known, data is available on who is responsible for the dam. The U.S. Army Corps of Engineers publishes and maintains the Low-head Dam Inventory and is committed to working with other federal, tribal, state, and private and non-governmental partners to understand what data is most needed in the inventory, best practices for gathering and publishing this information, and information most helpful to decision makers and people who live near or recreate in areas with low-head dams.

Please email us with this information at NID@usace.army.mil. To be added to the Low-head Dam Inventory, the following information will be needed:

• Verification that the dam meets the definition of a low-head dam
• Data showing the location of the dam, it's alignment on the water, and height
• Information about who is responsible for the dam

While publicly sharing a national dataset of low-head dams is a tremendous step forward, more work is needed to continue to improve available data, ensure low-head dams are maintained, and improve public awareness of these dams and safe recreation near them. The U.S. Army Corps of Engineers is committed to working with partners in dam safety, including federal, tribal, state, local, and non-governmental entities, to provide access to up-to-date information. While the U.S. Army Corps of Engineers maintains the Low-head Dam Inventory and completes some quality assurance on the data provided, the agency regulating the particular dam is responsible for the data and its correctness.