Frequently Asked Questions on Dams + Dam Safety

What is the definition of a dam?

Webster's Dictionary provides a very simple definition of a dam: a barrier built across a waterway to control the flow or raise the level of the water.

A much more detailed definition is given in the Federal Guidelines for Dam Safety, which are followed by all Federal agencies responsible for the design, construction, operation, and regulation of dams.

Any artificial barrier, including appurtent works, which impounds or diverts water, and which (1) is 25 feet or more in height from the natural bed of the stream or watercourse measured at the downstream toe of the barrier or from the lowest elevation of the outside limit of the barrier if it is not across a stream channel or watercourse, to the maximum water storage elevation or (2) has an impounding capacity at maximum water storage elevation of 50 acre feet or more. These guidelines do not apply to any such barrier which is not in excess of 6 feet in height regardless of storage capacity, or which has storage capacity at maximum water storage elevation not in excess of 15 acre-feet regardless of height. This lower size limitation should be waived if there is potentially significant downstream hazard.

Why are dams important?

Dams are important because they provide people with water to drink and provide water for industry, irrigation, fishing and recreation, hydroelectric power production, and navigation in rivers. They also reduce or prevent floods.

How many dams are there in the United States?

As listed in the 1995/1996 update to the National Inventory of Dams (NID), there are 75,187 dams in the United States. Of this total, more than 95 percent are owned by the States, local governments, industry, and individuals. For more information on the National Inventory of Dams, you can visit the United States Army Corps of Engineers web site address for this database at

I am downstream of a dam and concerned about my safety if there's a flood.
How can I find out who is responsible for the dam and what they're doing to make sure I'm not at risk?

There are a number of ways to determine who is responsible for operating and maintaining the dam. First, you can contact your State dam safety official (visit the web site of the Association of State Dam Safety Officials for a list:. Another option is to contact your representative to the Congress.

What is the highest dam in the United States?

The highest dam in the United States is Oroville Dam on the Feather River in California. The Oroville Dam, which was completed in 1968, measures 754 feet in height.

What was the worst disaster ever caused by a dam failure in the United States?

The worst disaster caused by a dam failure in the United States occurred in the Spring 1889 in Johnstown, Pennsylvania. After a night of heavy rain, the South Fork Dam broke, sending 20 million tons of water and debris down a narrow valley and onto Johnstown. The flood left over 2,200 dead and many more homeless. For more information on the Johnstown flood, you can visit the Johnstown, Pennsylvania homepage at .

How can I find out more about dam safety?

You can visit the list of publications published by FEMA on dam safety on this Web site. ASDSO also maintains a bibliography of dam safety-related publications. The bibliography can be viewed on ASDSO's Web site at .

How do dams work?

Dams store water in the reservoir during times of excess flow so that water can be released from the reservoir during times that natural flows are inadequate to meet the needs of water users.

What is the largest manmade reservoir in the United States?

Lake Mead, the reservoir for the Hoover Dam, is over 28 million acre-feet.