Hot - water fountains in the earth

On a bare, rocky patch of land sits a cone-shaped hump of rock with a hole in its top. Suddenly, with a hiss, a great, silvery spray of steam shoots up out of the hole. A geyser has erupted.

Geysers are the earth's hot-water fountains. Some geysers shoot out steam every few months. Others go off several times an hour. Some of the most famous geysers shoot steam more than a hundred feet (30 meters) into the air.

Geysers are found in groups in several parts of the world. They are near places where cold water from a river or lake drains down into the ground until it reaches hot rocks below the earth's crust. The hot rocks turn the water into steam. The steam pushes up through cracks in the earth and comes shooting out into the air. Sometimes the steam cools off before it reaches the surface. Then, hot water comes bubbling up out of the ground.

Water beneath your feet

All the earth's water isn't in seas, lakes, ponds, and rivers. A lot of it is beneath your feet - down in the ground.

When rain falls, much of the water seeps down through the soil. It keeps going until it reaches solid rock that it can't get through. Then it spreads out, filling every nook and cranny in the ground.

The top of this underground waters is called the water table. When there is a lot of rain, the water soon fills all the open spaces. Then the water table gets higher.

In some places the water table comes all the way to the top of the ground. Then, water bubbles out and makes a natural fountain called a spring. Sometimes a spring is the start of a river.

Underground water is cool and clean and good to drink. People often dig wells to get this water. There is some underground water almost everywhere in the world - even in deserts. But in a desert the water is very, very far down.

Holes full of water

A lake is the exact opposite of an island. An island is a piece of land that has water all around it. A lake, or a pond, is water that has land all around it. Some of the places we call seas, such as the Dead Sea, are really lakes, because there is land all around them.

Most lakes are just holes in the ground that are filled with water. Many such holes were dug by glaciers. Long ago, these huge rivers of ice flowed out of the north and covered many parts of the world. As the gigantic glaciers slid slowly along, they gouged out great pits and made valleys wider and deeper. Then, when the glaciers began to melt, the water filled up many of the holes, forming lakes.

Some lakes form when part of the earth caves in, leaving a hole. This happens mostly in places where the ground is limestone. Year after year, rain dissolves away the soft limestone. As the rainwater trickles through the limestone, underground caves and tunnels form. Finally, the tops of these tunnels cave in, leaving what is called a sinkhole. Rain, or water from underground springs and streams, fills the sinkhole and it becomes a lake or pond.

Part of a river can also become a lake. Sometimes a river deposits so much mud and sand that the water backs up and forms a natural lake. Or, people may make a lake by building a dam that causes the flowing water to spread out over the river's banks.

Some lakes were once volcanoes ! They formed when the craters of dead volcanoes filled up with rain water.

Flood

The water is rising ! The river is spreading into the streets of the town ! It's a flood !

Rivers often cause floods because of too much rain or the sudden melting of lots of ice and snow. A lot of the rain that falls on land runs into the nearest river. Water from melting ice and snow also runs into rivers. So when there is a long, heavy rain, or lots of melting ice and snow, tons and tons of water may pour into a river. Just as a bathtub will overflow it you keep running water into it, the river soon spills over its banks and floods the land.

Hurricanes and other bad storms sometimes cause floods along the seacoast. Strong winds push great waves far onto the land. Soon, much of the shore is under water

Where a river meets the sea

What has its head at one end and its mouth at the other end ?

A river ! The place where a river begins is called its head. And the place where it comes to an end, where it flows into a lake or the sea, is called its mouth.

At the edge of the sea, a river's mouth is often a sort of dumping place. As a river moves through the land, it tears sand and soil into it. The river carries all this sand and soil with it on its journey to the sea.

If there are no strong tides or big waves at the river's mouth, the soil and sand sink down to the bottom. As this soil and sand pile up in the riverbed, a kind of island forms in the middle of the river's mouth. Then the river has two branches that flow into the sea.

Slowly, the island gets bigger. In time, islands form in each of the branches. These islands split the river into still more branches. And after a long, long time, there is a great plain at the river's mouth, with many branches of the river running through it. This plain, usually shaped somewhat like a triangle, is called a delta. It gets its name from a letter of the Greek alphabet called "delta", which is shaped like a triangle.

The deltas of such rivers as the Mississippi, the Nile, and the Amazon are hundreds of miles (kilometers) wide. These deltas have been growing for thousands of years. They will keep growing for many years to come.

The story of a river

A river may begin as a trickle of melting snow, high on a mountaintop. It may begin as a trickle of underground water, bubbling out from under a rock on a mountainside.

The trickle winds down the mountainside, following the easiest path in and out among the rocks. It is so narrow you could step across it. Farther down the mountainside, it is joined by another little trickle. The two of them move along together, forming a wider, faster-moving stream,

Soil and stones, carried along by the rushing water day after day, year after year, cut a groove into the mountainside. The bottom of this groove is the bed of the stream. And the high sides of the groove are its banks.

One after another, more trickles join the stream and it grows wider. Now it is a river, fast and wide, rushing down the sloping mountainside.

In one very steep place, the fast-moving rivers has worn away the soft rock. Only bumps of hard rock are left. These rocks stick up out of the riverbed. The river swirls and foams around them. This part of the river is called the rapids.

Not far from the rapids, the mountainside ends in a cliff. The rushing river hurries to the edge of the cliff and falls hundreds of feet (meters) in a roaring, tumbling, splashing, waterfall.

The bottom of the waterfall is near the bottom of the mountain. The land there slopes very gently, so the river moves more slowly. The river leaves the mountain behind and flows out onto a plain. There, it moves even more slowly, because the plain is almost level.

Other rivers from other mountains join the first river. Together they become a great, broad river that flows slowly across the plain on its jouney to the distant sea

Why the sea is salty

You could be out in the middle of the ocean - surrounded by thousands of miles (kilometers) of water - and not have a drop of water you could drink. For seawater is full of salt. If you did drink it, it would simply make you more thirsty.

The sea is salty because rivers dump salt into it. All the rivers that flow down mountainsides and over the land tear loose tons and tons of minerals. Most of these minerals are different kinds of salts. The rivers carry these salts to the sea.

There's never enough salt in a river to make the river water taste salty. But rivers have been dumping salt into the sea for millions of years. By now, there is enough salt in the sea to cover all the land on earth with a layer of salt hundreds of feet (meters) deep !