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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

ablation

The process by which ice and snow waste away owing to melting and evaporation.

abutment

The part of a valley or canyon wall against which a dam is constructed. Right and left abutments are those on respective sides of an observer looking downstream.

abutment seepage

Reservoir water which moves through seams or pores in the natural abutment material and exits as seepage.

acre-foot (acre-ft, ac-ft)

The volume of water needed to cover one acre to a depth of one foot.

active conservation storage

The portion of water stored in a reservoir that can be released for all useful purposes such as municipal water supply, power, irrigation, recreation, fish, wildlife, etc. Conservation storage is the volume of water stored between the inactive pool elevation and flood control stage.

active (usable) storage capacity

The total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.

aeration zone

A portion of the lithosphere in which the functional interstices of permeable rock or earth are not filled with water under hydrostatic pressure. The interstices either are not filled with water or are filled with water that is no held by capillarity.

afterbay

The tail race of a hydroelectric power plant at the outlet of the turbines. The term may be applied to a short stretch of stream or conduit, or to a pond or reservoir.

agglomerate

An ice cover of floe formed by the freezing together of various forms of ice.

albedo

The portion of incoming radiation which is reflected by a surface.

ALERT Flood Warning System

A cooperative, community-operated flood warning system. The acronym stands for Automated Local Evaluation (in) Real Time.

alluvial

An adjective referring to alluvium.

alluvium

Sediments deposited by erosional processes, usually by streams.

anabranch

A diverging branch of a river which re-enters the main stream, characteristic of braided streams.

anchor ice

Submerged frazil ice attached or anchored to the river bottom, irrespective of its formation.

anchor ice dam

An accumulation of anchor ice which acts as a dam and raises the water level.

annual flood

The maximum discharge peak during a peak during a given water year.

antecedent precipitation index (API)

An index to soil moisture within a drainage basin.

antecedent precipitation index (API) method

A statistical method to estimate the amount of surface runoff which will occur from a drainage basin from a given rain storm based on the antecedent precipitation index, physical characteristics of the drainage basin, time of year, storm duration, rainfall amount, and rainfall intensity.

aquiclude

A formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring.

aquifer

A geologic formation which contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.

aquifuge

A geologic formation which has no interconnected openings and cannot hold or transmit water.

arch dam

A concrete arch dam is used in sites where the ratio of width between abutments to height is not great and where the foundation at the abutments is solid rock capable of resisting great forces. The arch provides resistance to movement. When combined with the weight of concrete (arch-gravity dam), both the weight and shape of the structure provide great resistance to the pressure of water.

area of influence

The area covered by the drawdown curves of a given pumping well or combination of wells at a particular time.

area-capacity curve

A graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.

arid

An adjective which describes regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable without irrigation.

arroyo

A water-carved channel or gully in arid country, usually rather small with steep banks, dry most of the time, due to infrequent rainfall and the shallowness of the cut which does not penetrate below the level of permanent groundwater.

artesian

Used to describe a well in which the water level stands above the top of the aquifer tapped by the well. A flowing artesian well is one in which the water level is above the land surface.

artesian well

A well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called a flowing well.

attenuation

The process where the flood crest is reduced as it progresses downstream.