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Pebbles and gravel
Fast-flowing river
Layers of hard rock form ridges on the river bed.
Rain shadow
The way that a mountain range blocks the path of air and forces it to rise and release its moisture on the windy side, leaving very little moisture left on the side sheltered from the wind. That side is the rain shadow side of the mountain.
Rapids
Fast-flowing stretches of water formed where the river surface breaks up into waves because rocks are near the surface. For this to happen, the river must flow over beds of rocks that are jutting upward.
 Rapids—Ledges of sloping rock make a series of rapids.
Many rapids are used as recreational areas for white-water rafting. (See also: Cataract and Fall line.)
Reservoir
An artificial lake created by building a dam across a river. Some reservoirs are called lakes because of their size.
River flows naturally vary through the year, so that there may be more water than can be used at one time of year and too little at another. During periods
of heavy rainfall rivers may also swell, burst their banks, and flood, leading to widespread destruction. Reservoirs are built to control the flow of water in a river to prevent flooding and to ensure a reliable flow of water through the year.
Reservoirs are used to provide drinking water, for irrigation water, and to generate hydroelectric power.
Reservoir sites have to be chosen with care because they take up a large amount of land that cannot then be used for other purposes. Ideally, sites are chosen where a
Reservoir
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