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Levee
Levee
A natural or artificial embankment or dike that runs along both sides and parallel to a river channel. The levee traps the river into a narrow channel at all times except during the highest floods.
Natural levees are built up during floods as rivers flow out from their channels. Water, including much suspended silt and sand, flows from the fast-moving channel to the relatively slow- moving areas of the flood plain. As a result, the silt and sand are quickly dropped and build up into long ridges. (See also: Groin.)
Lock
A gated device for allowing boats to navigate around weirs, barrages, and other obstacles along a river.
Melting point of ice
(See: Freezing point of water.)
Moisture
The amount of water held in the air as invisible water vapor. In this state water is a gas.
The amount of moisture in the air varies with the temperature of the air. Hot air can hold more moisture than cool air.
For example, when air flows toward a mountain, it rises up the mountain side and cools. Because the cooler air holds less moisture, it starts to release surplus moisture as droplets or ice crystals, and
Scouring of the outside of a bend produces a steep slope called a river cliff.
this produces a cloud. When the droplets group together, they become rain or snow.
O
Oasis
A reliable natural spring in a desert.
Oases are fed by springs
flowing from rocks. The source
of the spring may be hundreds of kilometers away from the spring
in a place where water is more plentiful, such as a mountain range.
Many oases are very small, but some can be hundreds of square kilometers in area.
M
Meander
A part of a river with regular bends.
A meander continually
changes shape as the fast-
flowing current of water swirls around the outside of the bend, eroding the outside bank. At the same time, slack water on the inside of the bend lets silt drop out. Because erosion occurs on one side of a meander bend and silt falls on the other, the channel stays the same width even though the bend keeps moving across a flood plain. (See also: Cliff, river cliff.)
The pattern, shape, and size of meanders vary with the steepness of the river course and the amount of water flowing.
Meanders can develop very pronounced loops called oxbows.
 Meander—A section of river showing how water flows around the outside of each bend, scouring on the inside and transferring material to the inside.
Corkscrew flow of water carries the scoured material to the slack waters of the inside of the bend, where they settle to the bottom.
Meanders increase the
length of a river and make river transportation less efficient. Many meanders have been cut through to create shortcuts on the world’s largest navigable rivers, most notably on the Mississippi River below Greenville, Mississippi. (Compare with: Braided river.)
 Meander—Meanders are the natural winding of a river over a flood plain. The river cliffs are on the outside of each bend. Notice how the insides of the bends have plants growing on them. That is where the sand and mud have recently been laid down. (Inset) The location of the main current at each bend.
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