Why are dams sometimes necessary? How do locks and dams work? What are some of the effects of a lock and dam system on the river's environment?

Locks and Dams
How does a lock and dam make an unnavigable part of a waterway navigable?
Peggy finds out how the flow of the mighty Mississippi can be altered with the push of a button.
Segment length: 8:00


Much of our folklore has explored the possibility of "escaping" on the river. Before the advent of the car, many a youth dreamed of traveling cross-country by water. Today there are 25,500 miles of navigable inland waterways in the United States; the Mississippi River, featured in the segment, accounts for approximately 9,000 miles of this system.

Equally appealing to youth is building a pint-size version of a dam by piling up sticks and debris to alter the flow of rainwater that is running down street-side curbs toward sewer drains. Dams are fascinating and useful; ruins of the world's oldest dam in Egypt along the Nile date to 2,700 B.C. (It was 37 feet high and 348 feet long.) More recently, the United States has built more than 5,000 dams, Japan has built about 2,000, and India has more than 1,000.

Universally, humans need water, but nature delivers water in imperfect amounts due to climate and weather. (Humans also contribute to problems of water availability; for example, deforestation can create desert-like climates in places where water was once abundant.) Some areas have too much water, some not enough, and some have both conditions at different times. Because of this irregularity, dams can be built to make water supplies more useful. Dams have many functions: diversion dams control the supply of water to prevent floods; storage dams reserve water for use during dry periods; power dams generate electricity; and navigation dams provide navigable waters.

Some river areas may be unnavigable by commercial boats and barges because of shoals, rapids, waterfalls, or low water. The riverbed itself may change in elevation, which may prohibit reliable and economical navigation. Finally, some rivers have been dammed. A lock can help make navigation possible in each of these situations.

The technology of locks looks complex, but the principle is simple: The river is an inclined plane whose water moves in and out of locks by gravity. Think of locks as a flight of "water stairs" going up and down a hill. Water is drained from the first lock (using gravity) until the water level is even with the second one. The downstream gate is opened to allow the vessel into the lower lock, and the process is repeated. The lifting and lowering of vessels, some weighing up to 60 tons, is done without a great use of energy.


1. Dams and the resulting reservoirs change wildlife habitats. Are all the environmental changes negative?
2. How important is commercial navigation by water in the U.S.? How does it compare to trucking and railroad transportation? Are some products not suited to waterway navigation?
3. If a dam were to be built in your area, how would your economy be affected? What kinds of jobs would be available?


floodplain the relatively broad, flat valley floor built up by an active river and periodically submerged with floodwater
gravity the force that tends to pull any two objects together
lock a short channel or a waterway divided into steps by watertight gates at either end
Pascal's law a law stating that a confined fluid transmits externally-applied pressure uniformly in all directions, without change in magnitude
watershed the area drained by a river or stream into the place where a dam will be built


Adler, J. (1990) Troubled waters. Newsweek (Apr 16): 66-80.

Ardley, N. (1990) How we build dams. Ada, OK: Garrett Educational Corp.

Nelson, S.B. (1983) "Water engineering." In Standard handbook for civil engineers, ed. F.S. Merritt. New York: McGraw-Hill.

Old man river. (1988) St. Paul, MN: U.S. Army Corps of Engineers.

Skorupa, J. (1991) The problem with dams. Popular Mechanics (Dec): 106-107.

Additional sources of information:

Iowa Department of Transportation
River Transportation Division
5268 NW 2nd Ave.
Des Moines, IA 50313
(515) 281-4292

U.S. Army Corps of Engineers
1421 U.S. Post Office & Custom House
St. Paul, MN 55101

Community resources:

State Department of Natural Resources
Civil engineer

Main Activity

Under Pressure
Pascal's law is demonstrated in this water-pressure demonstration.

A cubic foot of fresh water weighs 62.4 pounds. Water weighs more than a heavy wood such as oak, but half as much as bricks. The point is, of course, that as the elevation of water behind a dam is increased, the height and density of it causes high pressure at the bottom of the dam. In this activity, you will observe what happens to the flow of water when it is under pressure.


  1. Punch three holes horizontally at the bottom of one can, about 1/2" to 3/4" apart. Cover the holes with adhesive tape. Ask your students to predict what will happen when you fill the can with water and remove the tape: Will any of the streams be longer than the others?
  2. Punch three holes diagonally in the side of another can. Cover the holes with adhesive tape. Ask your students to predict what will happen when you fill the can with water and remove the tape: Will any of the streams be longer than the others?
  3. Fill both cans with water, and then tear off the tape on the can with the horizontal holes. Observe what happens.
  4. Now, remove the tape from the can with the diagonal holes. Observe what happens.


1. What happened when the tape was removed from the can with the horizontal holes? (All the streams are the same length because, according to Pascal's law, water pressure at a given depth is the same in every direction.)

2. What happened when the tape was removed from the can with the diagonal holes? (The longest stream of water shoots from the bottom hole because the deeper the water, the greater the pressure.)

3. Would this same principle of water pressure work with a balloon? Try it!

4. What household devices or appliances use the principle of water pressure to function?

Map the largest river near your school. Are there locks and dams on the river? Report on the geography and geology of the river, paying specific attention to the location of the locks and dams. How far could you go on an inland-water journey?

A cubic foot of fresh water weighs 62.4 pounds (1 kilogram per liter). To understand the concept of how dense water is, calculate the weight of a waterbed. Call a waterbed store to find out the amount of water contained in a super-single-size waterbed, a queen-size waterbed, and a king-size waterbed.

Do some research about some of the world records for subjects pertaining to locks and dams: the dam with the greatest volume; the largest dam; the highest dam; the longest dam; the largest reservoir; the earliest-known reservoir; the most damaging dam disasters; the greatest number of locks on any one waterway; the largest set of locks; the deepest locks.

Visit a beaver dam. What similarities are there to dams constructed by humans? Why do beavers construct dams? What materials do beavers use?

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