How does moving water change the earth's surface? Because gravity makes water run downhill, water can move materials from one place to another. As water flows over the earth's surface or within the earth, it carries materials from higher to lower places. Materials such as sand and clay are suspended in the water and carried in suspension. The amount of material and the size and weight of the pieces that water can carry depend on how fast the water is moving. The faster water moves, the more material and the larger and heavier the pieces it can carry. A stream flowing at a speed of 0.5 km per hour can carry fine sand. But a stream flowing 5 km an hour can roll stones the size of hen's eggs. Rushing mountain torrents can move huge boulders. Whenever water slows down, it drops part of the load that it has picked up. The materials settle to the bottom of the water as sediment. Water sorts the materials that it carries according to their size and weight. Large, heavy pieces can be carried only while the water is moving fast. As it begins to slow down, it drops the largest and heaviest pieces first. Then as it flows more slowly, it drops the smaller and lighter pieces a little farther along. Fine mud, or clay will not settle until the water has been still for some time. Water not only moves rocks and soil but it also digs valleys. A valley begins as a narrow groove in the earth through which a little stream flows. If the water runs rapidly, the stream cuts deeper and deeper. As long as it is cutting deeper, the valley remains narrow with steep sides. Such a valley has a V-shape and is called a young valley. After a time, the streams stops cutting downward so rapidly and moves more slowly. Then the stream begins to swing from side to side as it moves along. This cuts away the sides of the valley and widens its bottom, or floor. This time, it acquires a U-shape and is called a mature valley.

      During floods, a stream overflows its banks. In a mature valley or an old valley, the water spreads over the whole wide valley floor. The level ground covered by water is called a flood plain. When the flood has passed, the water flows slowly off the land. Then eroded materials such as clay and humus are left behind as sediment. Usually, flood plains are fertile from land. From time to time they recieve new deposits of soil that has been washed off higher ground.

     Moving water is not the only factor involved in shaping the surface of the earth. Wind, moving glaciers also have effects on the sorting out of materials that cover the surface of the earth. The movement of the earth's crust also plays a major role in dictating the surface features of the earth --- earthquakes, faulting, folding, vulcanism, etc. These processes of shaping the surface of the earth may take hundreds of years as in erosion or in just a moment as in a volcanic erution. The process either builds up or level off land formations through time or almost in an instant. Every minute of the day, though we may not notice it, something is happening to the earth's surface that makes the present condition utterly different from the condition maybe a second ago. Truly, ours is a dynamic earth.