Fresh Water

* Which is the world's longest river?

 The Nile in Africa and the Amazon in South America are the world's longest rivers. The Nile is 4,412 miles long, and the Amazon is 4,000 miles long. It is difficult to give an exact length because these huge rivers split into tiny tributaries near their souces, and some of these dry up or change course.

 The Amazon is by far the largest river because it carries more water than the Nile, Mississippi, and the Yangtze put together. The Amazon is 87 miles wide at its mouth, and up to 100 yards deep in places.

* How does a river begin?

 most of the rainwater that falls on the land returns to the sea by way of rivers. Rainwater first soaks into the ground, but once the ground is saturated, the excess water begins to run off in small streams. More water comes from melting ice and snow. Small streams gradually merge and become bigger, until they eventually form a river. The tiny streams that are the source of a river often begin in damp areas such as swamps or bogs, or may flow out of a pond or lake. Usually they are in hills or mountains where the rainfall is heaviest.

* What is a delta?

 As a river reaches the sea, its flow becomes slower, forming an area known as a delta. As the water slows, the tiny particles of silt carried by the current settle to the bottom, forming mud banks. These make the riverbed shallower, so the water spreads out of form a wide estuary. In large rivers like the Eanube, islands form among these exposed mud banks, and the river water trickles through a maze of small waterways. This region, which is rich in plants and other wildlife, forms the river's delta.

* Uphill Flow!

Water always flows down a river, but occasionally the flow is reversed. This happens only in very large rivers, when very high tides overcome the normal river currents. In narrow parts of the river valley, the water begins to pile up, and eventually a wave called a tidal bore passes back up the river, sometimes for a great distance. This happens in the Amazon in South America, where there is a bore as high as 5 yards.

* What is a spring?

Many types of rock contain tiny holes like a sponge, in which rainwater collects. The water gradually sinks down and eventually flows out lower down. Water stored in such porous rocks sometimes bubbles to the surface in springs. They are usually found near the foot of hills. Spring water is often used for drinking because as it passes through rock, any harmful substances or germs are filtered out. On more level ground, spring water is found by digging a well to reach the underground water. Small outflows of water can lead to the formation of isolated bogs or marshes.

* How are limestone caves formed?

 Water can actually dissolve certain types of rock, eventually forming underground streams and caves.

 Carbon dioxide in the air dissolves in rainwater to form a very weak acid, which can slowly dissolve soft limestone rock. Over thousands of years, water seeping into cracks in limestone rock dissolves so much that the cracks become holes. Slowly the holes grow even larger and form caves. These may be many miles in length and very deep. Sometimes the roofs of huge caves collapse, forming narrow valleys or gorges.

* Can water wear away solid rock?

 Water itself cannot wear away rock, but tiny particles of grit carried in the water can eventually wear away the hardest rocks. It is this continuous wearing process that cuts valleys through mountains and hills. The faster the water flows, the more the grit it carries wears away at the rock. You can see the same process in action along the coast. There, rocks are worn away by the action of sand dashed against them by the waves.

* Which is the world's highest waterfall?

 The Angel Falls in Venezuela, South America, fall 2,894 feet in a single drop. Much more water flows over Victoria Falls in southern Africa and Niagara Falls on the U.S. -Canadian border, but these falls are not as high.

 Waterfalls occur when a river flows over hard rock lying over softer rock. The hard rock forms a 'lip' for the waterfall, and the water wears away the softer rock below, forming a step that gradually becomes deeper.