The text on this site is presented as an archival version of the script of "Ocean Planet," a 1995 Smithsonian Institution traveling exhibition. The content reflects the state of knowledge at the time of the exhibition, and has not been updated.


There are over 75,000 dams in the United States, and more are planned. Why? We divert river water to meet household needs, irrigate fields, supply factories, control flooding, generate power, permit barge traffic deep inland, and make lakes for boating and fishing §.


Dams decrease the natural runoff from watersheds. Less sediment washes downstream to replenish coastal wetlands and beaches. Salt levels rise at river mouths, as less fresh water flows out. Dams interfere with migration routes and spawning grounds, and water released after long periods behind dams is often oxygen- poor and polluted §.

Bonneville Dam, completed in 1938
Before the 56 Columbia River dams were built for irrigation and hydroelectric power, 16 million wild salmon swam upriver each year. Today, streams and tributaries yield only 2.5 million salmon, most bred in hatcheries §.
photo © Gary Braasch/Woodfin Camp & Associates

What lowered water levels do

San Joaquin/Sacramento Delta, San Francisco Bay
More than a hundred dams and water-diversion systems built to provide irrigation and reclaim cropland have transformed the watersheds that feed San Francisco Bay. Water withdrawals have depleted the San Joaquin River's flow by up to 90 percent §.

Salinity has spread into the delta §. Combined losses of salmon, striped bass, shad, and other estuarine fisheries have exceeded $3 billion. Control structures have been built to reduce the amount of salt water entering the delta.
photo © California Department of Water Resources

Aswan High Dam, Nile River, Egypt, 1983
In the early 1960s Egypt dammed the Nile to generate electricity and provide year-round irrigation for agriculture §. During droughts, the Aswan High Dam diverts up to 95 percent of the Nile's normal water flow, holding back silt §.

The dam has deprived the Nile Delta and Mediterranean shores of more than 1 million tons of nutrient-rich silt §. Sardine and shrimp fisheries in the southeastern Mediterranean have declined sharply.
photo © Robert Caputo

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San Francisco Bay fish win one

Early in 1993, California and federal authorities imposed a new balance among the water demands on the enormous San Joaquin- Sacramento Delta that feeds fresh water into San Francisco Bay.

San Joaquin/Sacramento Delta, San Francisco Bay
New regulations sharply reduce pumping from the estuary system during fish migrations.
photo © California Department of Water Resources

Water savers save water for wildlife

Conserving water frees up more water for wildlife. Water- efficient toilets, shower heads, and faucets reduce domestic water consumption--a good thing, since flushing, bathing, and hand-washing use more than three-quarters of the water in a typical American home.

Traditional toilets turned in under a rebate program, Los Angeles, 1993
Although Los Angeles County spent $100 for each turned-in toilet, the state will save millions of gallons of water as residents switch to low-flow toilets that use less than 2 gallons per flush.
photo © Rick Rickman/National Geographic Society

Field flooders turn the tap down

Above-ground irrigation systems waste nearly two-thirds of the water they use because of evaporation and seepage. Since nearly 70 percent of water used in the U.S. goes for agriculture, that's a lot of water lost. Drip irrigation from buried plastic tubing takes water directly to roots, uses far less water than conventional field-flooding, and minimizes salt build-up from evaporation that can ruin traditionally irrigated fields.

Subsurface drip irrigation developed by USDA scientist Claude J. Phene
Although drip irrigation systems are expensive to install, they are a good investment. Farmers have doubled their yield with only half the water other growers use.
photo © James A. Sugar

More Information:

Ocean Planet Exhibition Floorplan

gene carl feldman ( (301) 286-9428
Judith Gradwohl, Smithsonian Institution (Curator/Ocean Planet)