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.
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.
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.
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.
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.
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.
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.
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.