Dams are structural barriers built to obstruct or control the flow of water in rivers and streams. They are designed to serve two broad functions.

Darcy, Henry

No other name is more closely associated with the field of hydrology than that of Henry Darcy. His expression for the movement of water through an aquifer, known as Darcy's Law, is still used today.

Data, Databases, and Decision-Support Systems

Databases are critical to the successful use of computer-based models that help to identify, compare, and evaluate various impacts of alternative management policies for specific watersheds. Menu-driven, graphics-based computer programs that permit the interactive use of these impact prediction models and their databases support the iterative (sequentially repetitive), explorative, participatory and adaptive decision-making processes that typify water resources management.

Davis, William Morris

William Morris Davis is a major historical figure in geomorphology, the scientific study of landforms. Davis is especially known for his theory of landscape development—called the geographical cycle—that was the leading geomorphic theory from 1890 to 1950.

Demand Management

Demand management is the purposeful and beneficial manipulation of the level and timing of water usage. Demand management deploys various techniques for conserving water and improving the efficient use of water by end users.


Because only 1 percent of the Earth's water is fresh, it is useful to utilize the oceans as a means of supplementing the fresh-water supply. To be potable (drinkable), however, salt and other chemicals must first be removed from the sea water.

Desert Hydrology

Deserts are arid regions, generally receiving less than 25 centimeters (10 inches) of precipitation a year, or regions where the potential evaporation rate is significantly greater than the precipitation. In most cases, deserts possess a high average temperature with large differences between daytime and nighttime temperatures.

Developing Countries, Issues in

Water resource issues and problems in the world's developing countries, or lesser developed countries, present special management challenges. These issues and problems include inadequate drinking-water supply and sanitation facilities, water pollution, floods, the siltation of river systems, and the management of rivers and large dams.

Douglas, Marjory Stoneman

Marjory Stoneman Douglas, until her death at 108, remained devoted to environmental activism to protect and restore the Everglades. Here she poses at age 96 with her best-known book.

Drinking Water and Society

The course of world society in the twenty-first century is likely to be substantially influenced by a single resource: drinking water. The first and most obvious fact is that water is an absolute necessity.

Drinking-Water Treatment

Public water systems vary in the treatment of water that is delivered to the consumer, depending on whether the system uses a groundwater source or a surface-water source. Some public water systems, primarily those that utilize groundwater wells, deliver untreated water directly to the customer's tap.

Drought Management

Drought is a subtle, insidious natural hazard that is a normal part of the climate of virtually all regions of the world. Its occurrence results in a myriad of economic, social, and environmental impacts in developed as well as developing nations, although the characteristics of its impacts differ considerably between the two settings.

Earle, Sylvia

Sylvia Earle's achievements as an oceanographer have earned her nicknames such as "Her Deepness" and "The Sturgeon General." Earle was one of the first scuba divers to explore underwater habitats, and she made what is still the world's deepest solo dive in 1979, when she descended 380 meters (1,250 feet) in a pressurized garment off the coast of Hawaii.

Earth: The Water Planet

In Earth's solar system, evidence of subsurface ice, ancient valley networks, and even an ancient ocean occurs on the planet Mars. Water ice and perhaps liquid water occur beneath the frozen surfaces of three of Jupiter's moons, although currently none have free surface waters.

Earth's Interior, Water in the

Water is found in different physical states at different levels below the Earth's surface. At shallower depths, it is a liquid occurring in fractures and in the pore spaces of rocks and unconsolidated materials.

Ecology, Fresh-Water

Streams, lakes, and wetlands differ profoundly from one another in the conditions they provide as habitats for biological communities. Fundamental characteristics of standing water (a lentic system) or flowing water (a lotic system), the dynamics of its interaction with adjacent land and vegetation, and seasonal fluctuations in water conditions determine characteristic biological assemblages.

Ecology, Marine

Marine ecology describes the interactions of marine species with their biotic (living) and abiotic (nonliving) environments. The biotic environment includes interactions with other living organisms.

Economic Development

Economic development occurs with the expansion of the economic base of a community, region, state, or nation through the efficient allocation and use of available resources. In general, economic development is any activity that results in additional jobs and income.

El Niño and La Niña

In 1982 and 1983 and again in 1997 and 1998, well-recorded El Niño events turned the world's weather upside down. The west coasts of the United States and Peru received record rainfall and flooding, while hurricanes raged in Mexico, Hawaii, and Tahiti.

Endangered Species Act

The Endangered Species Act (ESA) has been described as the most comprehensive legislation for the preservation of endangered species ever enacted by any nation. It is also one of the United States' most controversial natural resource laws.

Energy from the Ocean

The Earth's oceans could one day provide enough energy to power homes and businesses. Technologies have been developed to harness energy from tides, waves, temperature gradients, ocean currents, ocean winds, and salinity gradients.

Environmental Movement, Role of Water in the

Attitudes toward nature and viewpoints on the human species' relationship with nature have evolved over the course of history. Societies have always had to deal with environmentally related problems.