Reservoirs, Multipurpose

A reservoir is the artificial body of water that forms adjacent to a storage dam. Many of the modern reservoirs that operate today in unison with dams serve two or more purposes.

Rights, Public Water

Two interrelated aspects of water law are important for protecting the public's interest in water. First, the general public has rights to use water in place (without diversion).

Rights, Riparian

Water law today is primarily a product of statutes enacted by legislatures and regulations issued by implementing administrators, although reviewing courts remain important in the system. Modern law is being increasingly accommodated to ecosystem values—rather than solely to human demand—in establishing a "law of reason" for all water.

River Basin Planning

The concept of coordinated planning of water resources throughout a river basin dates back to the late nineteenth century. In the 1870s, John Wesley Powell recommended using major river basins as administrative units in the The Tennessee Valley Authority (TVA) in the southeastern United States employed thousands during the planning and construction phases of its major water projects.

Rivers, Major World

The size and importance of the world's rivers are measured in terms of discharge and length. A river's importance may also be measured in terms of local and regional water availability and population.

Runoff, Factors Affecting

Rain falling on the landscape may flow quickly over soil or rock surfaces as runoff to stream channels. Alternately, some water may flow more slowly downslope toward streams within the soil.

Safe Drinking Water Act

The Safe Drinking Water Act of 1974, amended in 1986 and 1996, is the legal basis for regulating public drinking water in the United States. Its passage was spurred largely by mounting public awareness about contaminants in the environment, and concern for the safety of drinking water.

Salmon Decline and Recovery

Salmon are indigenous to both the east and west coasts of North America. There are several species that inhabit western waters, with chinook salmon being the most abundant.

Sea Level

Sea level is defined as the height of the sea surface above an equipotential surface, called the geoid. The geoid is where the sea surface would come to rest in the absence of tides, water density variations, currents, and atmospheric effects.

Sea Water, Freezing of

The density of ocean water is determined by its salinity (or salt content) and temperature. The saltier and/or colder the water is, the denser it is.

Sea Water, Gases in

Gases in sea water reveal the ways in which a variety of physical, chemical, and biological processes interact in the oceans and coastal environments. A series of reactive trace gases found in sea water include methane, carbon monoxide, nitrous oxide, hydrogen sulfide, and hydrogen.

Sea Water, Physics and Chemistry of

Most people come in contact with the ocean only near its surface, and usually near its edges. In the huge part of the ocean that remains hidden, sea water is salty, cold, dark, and deep.

Security and Water

Complex relationships exist between water and the security of individuals, communities, nations, and the global community as a whole. The vital role of water in daily life and economic activity underscores its importance to a secure and stable world.

Senses, Fresh Water and the

The qualities of natural fresh water that can be detected by human senses are derived from various naturally occurring chemical, mineral, and bacterial sources. These constituents affect the way in which the human senses of sight, smell, taste, touch, and hearing perceive water and judge its aesthetic quality.

Septic System Impacts

When a toilet is flushed in a town or city, the wastewater flows through sewer pipes and eventually ends up at a treatment plant. Microorganisms that could be harmful to human health or the environment are removed from the water before it is released.

Solar System, Water in the

Liquid water is an essential ingredient for all life on Earth. With 70 percent of its planetary surface covered by water, Earth is unique in the solar system.

Sound Transmission in the Ocean

The speed of sound depends on the medium through which sound waves propagate. The speed of sound differs in air and water, with sound waves traveling faster in water.

Space Travel

Water forms Earth's oceans, lakes, and rivers, and is also present in the atmosphere. All Earth's creatures rely on water to sustain life.


Sports are activities involving physical exertion and skill in such areas as strength, speed, stamina, and dexterity in which an individual or team competes against another for entertainment of the participants or an audience.


A spring is a location where groundwater naturally emerges from the Earth's subsurface in a defined flow and in an amount large enough to form a pool or stream-like flow. Springs can discharge fresh groundwater either onto the ground surface, directly into the beds of rivers or streams, or directly into the ocean below sea level.* Springs form the headwaters of some streams.

Stream Channel Development

The geometry of a stream channel is controlled by both water and sediment movement, which reflect regional climate, geology, and human land use in a given drainage basin.

Stream Ecology, Temperature Impacts on

Water temperature has direct and indirect effects on nearly all aspects of stream ecology. For example, the amount of oxygen that can be dissolved in water is partly governed by temperature.