Evaporation

Oceans hold 97% of the water on the earth. The second largest amount of water, slightly over 2%, in frozen in huge, moving bodies of ice, called glaciers. The remaining water, less than 1%, is found flowing underneath and on top of the earth's surface. This 1% of water, a seemingly small amount, is the source of almost all of the water people every day.

As the sun beats down upon the earth, land surfaces absorb the heat and quickly release it back into the air again. Water, on the other hand, has a larger capacity than land for absorbing heat. Heat is not released into the air as quickly, which helps to keep the earth's atmosphere from becoming too hot. Many of the other planets in our solar system have wide ranges of temperature -- at times varying by hundreds of degrees. Water in the earth's atmosphere and on its surface helps prevent this from happening on our planet.

Surface water

Water in oceans, rivers, and lakes -- is an easily observed part of the hydrologic cycle. We can see surface water as it flows across the land. In fact, surface water often changes the land around us.

Flowing surface cuts into the land and erodes, or wears away, rock and soil. Pieces of eroded rock and soil, now called sediment, may be carried away by the water and deposited else and rolling it along ocean, lake, and river bottoms. Lighter sediment is carried suspended in the water. Sometimes so much sediment is suspended that the water appears cloudy. Water also moves sediment by dissolving it completely.

Each year oceans move billions of tons of sediment. Waves batter rocks and sand, changing cliff faces and beaches. Where there are significant changes in seasons, closely spaced, choppy winter waves crash onto beaches, narrowing them by pulling sand back into the ocean. The more widely spaced, lower waves of summer carry sand toward the shore, causing beaches to widen.

Powerful ocean currents carry sediment to new places, in turn building new land areas as the sediment is deposited. Many ocean side resorts and vacation homes are built on offshore ridges of sand known as barrier islands. Although these islands seem to be secure land, waves generated by powerful hurricanes can be strong enough to wash away buildings and sand.

Every time rivers overflow, they carry sediment. When flooding ends and rivers return to their normal channels, sediment is left behind. Over time and repeated flooding, some sediment deposits may form hills along the riverbanks. These hills, called levees, are natural barriers that can help prevent some future flooding.

Flat land alongside rivers is frequently covered by water when flooding occurs. These flat areas, called floodplains, are often covered with widespread layers of sediment that have been deposited by past floodwaters. Floodplain sediment can be many inches or feet thick. As a matter of act, much of the damage done inside homes that have been built on floodplains is caused by thick, muddy sludge left behind as rivers flow back into their channels.

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