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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).

main stem

The reach of a river or stream formed by the tributaries which flow into it.

major flooding

A general term indication extensive inundation and property damage. Usually characterized by the evacuation of people and livestock and the closure of both primary and secondary roads.

manometer

A device for measuring pressure by means of a column of liquid. The height of the liquid is direcly proportional to the pressure being measured.

mean areal precipitation (MAP)

The average rainfall over a given area, generally expressed as an average depth over the area.

mass curve

A graph of the cumulitive values of a hydrologic quantity (such as precipitation or runoff), generally as ordinate, plotted against time or date.

maxiumum spillway discharge Spillway discharge when reservoir is at maximum designed water surface elevation.

mean depth

The average depth of water in a stream channel or conduit. It is equal to the cross-sectional area divided by the surface width.

meander

The winding of a stream channel.

meander belt

The area between lines drawn tangential to the extreme limits of fully developed meanders.

meteoric water

Water derived from precipitation.

minor flooding

A general term indicating minimal or no property damage but possibly some public inconvenience.

moderate flooding

The inundation of secondary roads; transfer to higher elevation necessary to save property. Some evacuation may be required.

moisture equivalent

The ratio of

• the weight of water which the soil, after saturation, will retain against a centrifugal force 1,000 times the force of gravity, to
• the weight of the soil when dry.

This ratio is stated as a percentage.

moveable bed

A stream bed made up of materials readily transportable by the stream flow.

moveable bed stream

This type of stream is most common in the arid West, where steep slopes and lack of vegetation result in a lot of erosion. During a flood, a channel may be eroded more deeply, or it may become filled with sediment and move to a different location.

multipurpose reservoir

A reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control, power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc.