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Ribbon lake
dam can be built across a deep, narrow valley. This means that
the cost of building the dam is
kept as low as possible, while
the steep, narrow valley will use
up a minimum of land. A further advantage is that the surface area is small compared with the volume of water, so in hot weather the amount of water lost by evaporation is kept low.
Because reservoirs are regions of still water in the path of a river, when the river enters the reservoir, any sediment it is carrying will settle out. Over time this will fill in the lake. It is therefore important to build reservoirs on rivers that do not have a high sediment load.
Most reservoirs have a useful lifespan of fewer than 150 years. After this they contain too much sediment to work properly. Those reservoirs located on rivers with a high silt load may only last for a few decades.
 Reservoir—Reservoirs can be enlargements of existing lakes, or they can
be flooded river valleys. Reservoirs on flat land may need to be enclosed on all sides by a wall called a bund.
Ribbon lake
(See: Finger lake.)
River
A naturally winding channel
that drains surplus water from a drainage basin. (Small rivers are called streams and brooks).
At any one moment the world’s rivers carry only one-ten- thousandth of 1% of the water that exists in the oceans and in the atmosphere of the Earth. But
this amount, some 40,000 cubic kilometers a year, is usually enough to provide the world’s people with all the water they need.
A river is part of the water cycle. It carries the water that has fallen as rain or has melted from snow back to the oceans, where it evaporates to make rain once more. As this water flows back
to the sea, it has the energy to pick up and carry loose material. This material is called sediment. (See also: Abrasion; Alluvium; Attrition.)
As a river flows, it shapes land by cutting down into its bed. If no other erosion happened, as is the case when a river flows through
a desert, the river cuts a gorge. A river by itself cannot cut a valley, that is the result of a combination of river erosion and other processes such as landslides.
River
Nile
Amazon
Yangtze Mississippi Yenisey
Huang Ho (Yellow) Ob-Irtysh
Paraña Congo Amur
World’s longest rivers Drains into
Mediterranean Sea South Atlantic Ocean East China Sea
Gulf of Mexico
Kara Sea
Gulf of Chihli
Gulf of Ob
South Atlantic Ocean South Atlantic Ocean Sea of Okhotsk
Length (km)
6,650 6,400 6,300 5,971 5,540 5,464 5,410 4,880 4,700 4,444
 River—A river is a place where water flows in a channel. 34