Acid Mine Drainage

A high school freshman sits in a grass meadow where two mountain ridges slope down and join. As the young observer gazes across the landscape, he sees a scar on the neighboring ridge.

Acid Rain

In October 1998, U.S. Senator Daniel Patrick Moynihan testified before Congress on acid rain.

Agassiz, Louis

Jean Louis Rodolphe Agassiz has been called the "Father of Glaciology" and the "First Naturalist." One of the greatest contributors to the science of water, he discovered evidence of a time when the frozen state of water changed Earth's landscape: the Ice Age.

Agriculture and Water

Humans depend on water in many ways, well beyond the few liters needed daily for drinking. Water is also essential for the production of food.

Algal Blooms, Harmful

Single-celled algae are almost always present in sea water even if the water looks clear. When high concentrations of certain species of dinoflagellates are present, patches of water look red because these algae contain red pigments—hence the name "red tide." High concentrations of other algae The term "red tide" refers to different types of algal blooms, which can be various hues depending on the species and the photosynthetic pigments they contain.

Algal Blooms in Fresh Water

Aquatic ecologists are concerned with blooms (very high cell densities) of algae in reservoirs, lakes, and streams because their occurrence can have ecological, aesthetic, and human health impacts. In waterbodies used for water supply, algal blooms can cause physical problems (e.g., clogging screens) or can cause taste and odor problems in waters used for drinking.

Algal Blooms in the Ocean

The ocean, that vast body of water covering 71 percent of the Earth's surface, is divided into four major basins: the Pacific, Atlantic, Indian, and Arctic Oceans. These large basins are interconnected with various shallow seas, such as the Mediterranean Sea, the Gulf of Mexico, and the South China Sea.

Amphibian Population Declines

An unparalleled diminishment in populations is occurring worldwide in many species of amphibians (frogs, toads, and salamanders). Although there are various causes for declining amphibian populations, the most obvious is habitat destruction.


Aquaculture, a type of agriculture, is the practice of cultivating aquatic animals and plants in managed aquatic environments. Aquaculture in salt-water or marine environments is called mariculture.


The pleasure of viewing and contemplating aquatic species has its roots in antiquity. The ancient Egyptians, Romans, and other peoples kept fish in artificial pools.

Aquifer Characteristics

Groundwater is stored in the open spaces and fractures within geologic materials such as soil, sand, and rock that occur beneath the land surface. Aquifers are the geologic layers that are filled with water and that can transmit enough water to supply a well under normal hydraulic gradients.

Archaeology, Underwater

The development of lightweight sophisticated diving equipment since World War II (from 1939 to 1945) has made underwater archaeology a flourishing branch of archaeology. The science of underwater archaeology involves the recovery and study of submerged archaeological finds and sites, including ancient springs and wells, lakeside settlements, and marine sites such as sunken cities, harbors, and shipwrecks.

Army Corps of Engineers, U.S

The Continental Congress in June 1775 organized what later became the U.S. Army Corps of Engineers (ACE) when it authorized an engineer and two assistants to prepare fortifications for the Battle of Bunker Hill.

Artificial Recharge

Water-supply development is challenging. Increasing demands for water joined with concerns for environmental protection require a variety of new water management tools.

Arts, Water in the

From ancient times, in Western culture and worldwide, water has been an enduring theme in the arts. Water themes (including snow and ice) flow Sea monsters in literature often are exaggerations of naturally occurring creatures.

Astrobiology: Water and the Potential for Extraterrestrial Life

Astrobiology is a new interdisciplinary science that seeks to understand the origin, evolution, distribution, and future of life in the universe. As a fundamental requirement of living systems, water holds a special place in the conceptual framework of astrobiology.

Attenuation of Pollutants

The moment that pollutants in soils become dissolved in natural waters, their potential for transport is greatly magnified, as is the likelihood that people will ingest them. The primary health risk from many hazardous waste sites, dumps, septic tanks, factory outflows, and other pollution sources is the possibility that pollutants will be dissolved into groundwaters or surface waters, then ultimately reach drinking water.

Balancing Diverse Interests

Water is a natural resource critical to the environment. Water also is an economic resource critical to society.

Bays, Gulfs, and Straits

Bays, gulfs, and straits are types of waterbodies that are contained within a larger body of water near land. These three waterbodies are usually located at important points of human activities; thus, conflicts with nature and neighbors can result.


A beach is a dynamic environment located where land, sea, and air meet. It may be defined as a zone of unconsolidated sediment (i.e., loose materials) deposited by water, wind, or glaciers along the coast, between the low tideline and the next important landward change in topography or composition.


Biodiversity describes the variety of biological organisms in a given habitat, area, or ecosystem. It includes several components involving variation in species, ecosystems, and genetics.

Birds, Aquatic

Aquatic environments provide critical habitat to a wide variety of bird species. Some aquatic birds divide their time between aquatic and terrestrial environments, while others spend most of their lives in water, returning to land only to breed.