Alluvial: Applied to the environments, action, and products of rivers or streams. Alluvial deposits (alluvium) are silt, sand, and gravel material transported by a stream or river and deposited as the river floodplain.

Aquifer: A body of permeable rock, unconsolidated gravel, or sand that can store significant quantities of water, is underlain by impermeable material, and through which groundwater moves. An unconfined aquifer is one in which the water table defines the upper water limit. A confined aquifer is sealed above and below by impermeable material.

Available relief: The part of a landscape that is higher than the floors of main valleys. It is therefore available for destruction by the agents of erosion, controlled by the local base level. It is measured by the vertical distance between hilltops and valley floors.

Bankfull flow: Maximum amount of discharge (usually measured in cubic feet/sec.) that a stream channel can carry without overflowing. The water height at bankfull discharge is called the "bankfull stage."

Biological oxygen demand: Indicator of the polluting capacity of an effluent, where pollution is caused by the takeup of dissolved oxygen by microorganisms that decompose the organic material present in the effluent. It is measured as the weight (mg) of oxygen used by one liter of sample effluent stored in darkness at 20 degrees C. for five days.

Competence: The maximum size of rock particle transported by a particular flow of ice, water, or air. Ice has high competence because of its viscosity; flowing water has a lower competence, though this increases sharply as velocity increases. Wind has the least competence.

Critical erosion velocity: The minimum velocity of a flowing fluid required to entrain a particle.

Discharge: A measure of the water flow at a particular point, e.g., a river gauging station or sewage works. River flow may be expressed in cubic feet per second.

Drainage: 1. The passage of water over and through the land surface, ultimately toward the sea. 2. Process of removing the gravitational water from soil, using artificial or natural conditions, such that freely moving water can drain, under gravity, through or off soil.

Elevation potential energy (elevation head): The energy possessed by a mass, e.g., a body of water, by virtue of its being raised above a particular datum point, usually taken as sea level or local ground level. The energy may be released when the mass is allowed to fall to a lower level, and may be harnessed, e.g., in the case of water, by powering a turbine in a hydroelectric scheme.

Englacial: Contained within the interior of a glacier, as opposed to being at its base (subglacial) or on its surface (supraglacial). Normally the term is applied to meltwater or drift.

Erosion: 1. The part of the overall process of denudation that includes the physical breaking down, chemical solution and transport of material. 2 Movement of soil and rock material by agents such as running water, wind, moving ice, and gravitational creep (or mass movement).

Erosion rate: The rate at which geomorphological processes wear away land surfaces. Rates vary widely, depending on both processes and environments.

Field capacity: The water content that can be retained by a soil after excess moisture has drained freely away. It is usually measured as a percentage of the soil volume, or of the weight of oven-dry soil.

Floodplain: The part of a river valley made of unconsolidated river-borne sediment, and periodically flooded. It is built up of coarse debris left behind as a stream channel migrates laterally, and of fine sediment deposited when bankfull discharge is exceeded.

Flowmeter: An instrument for measuring the flow of liquids.

Geomorphology: The scientific study of the landforms of the earth's surface and of the processes that have fashioned them.

Glacial drainage channel (meltwater channel): Channel cut by the action of glacial meltwater or by water from an ice-dammed lake. Various types may be recognized, classified by the position of the channel with reference to the glacier, e.g., ice-marginal, englacial or subglacial. Usually these channels are steep-sided and flat floored.

Glacial limit: A line marking the furthest extent of a former glacial advance. It may be identified on the ground through the recognition of features associated with glacial margins, including lateral and terminal moraines, outwash spreads, marginal meltwater channels, and proglacial lakes.

Hydraulic head: In general, the elevation of a body of water above a particular data level. Specifically, the energy possessed by a unit weight of water at any particular point. The hydraulic head consists of three parts: the elevation head, defined with reference to a standard level or datum; the pressure head, defined with reference to atmospheric pressure; and the velocity head. Water invariably flows from points of larger hydraulic head to points of lower head, down the hydraulic gradient.

Load: Total amount of material carried by a stream or river.

Loess: Unconsolidated, wind-deposited sediment composed largely of silt-sized quartz particles and showing little or no stratification.

Misfit stream (underfit stream): Stream that is too small to have cut the valley it currently occupies.

Moraine: An accumulation of boulders, stones and other debris carried and deposited by a glacier.

Outwash: 1. The stratified sands and gravels deposited at or near to ice margins. 2 Meltwater escaping from the terminal zone of a glacier.

Proglacial: Applied to the area between a glacier and adjacent high ground. A proglacial lake is a body of water impounded in such an area and is often inferred for areas of Pleistocene glaciation from the evidence of strand lines, lake sediments, and overflow channels.

Rock flour: Finely ground rock debris produced chiefly by abrasion beneath a glacier. It may be removed by meltwater streams, which consequently develop a typically milky appearance.

Spillway: General term for a glacial drainage channel cut by water during glaciation, and normally including three varieties: (a) channels cut by water escaping from a glacially impounded lake; (b) channels cut by meltwater released by a decaying glacier; and (c) channels cut by a stream deflected by an advancing glacier.

Stage: The elevation of the water surface of a river relative to a fixed datum level.

Strand: A shore or beach, often used to identify shorelines of ancient glacial lakes.

Stream power: The rate at which a stream can do work, especially the transport of its load, and measured over a specific length. It is largely a function of channel slope and discharge and is expressed by: Power = (the specific weight of water) x discharge x slope. Streams tend to adjust their flow and channel geometry to minimize their power.

Till: Collective term for the group of sediments laid down by the direct action of glacial ice without the intervention of liquid water.