Table 13.1: (continued)# Name Continent Rate of Flow
m^3 /sApproximate
Length (km)
6 Huang He Asia 2,600 4830
7 Congo Africa 43,000 4630
8 Lena Asia 17,000 4400
9 Amur Asia 6,000 4350
10 Yenisei River Asia 20,000 4106Ponds and Lakes
Streamsandrivers, bydefinition, arebodiesofwaterthathaveacurrent; theyareinconstant
motion. Ponds and lakes, on the other hand, do not (Figure13.11). They are generally
bordered by hills or low rises, so that the water is blocked from flowing directly downhill.
They represent yet another important resource for humans and another area in need of
conservation.
Though the wordpondrefers to water that does not constantly flow downhill, there is
disagreement about the exact definition of a pond. It is generally agreed, however, that a
pond is a small body of freshwater. You probably wouldn’t need a boat to get across it, and
you might be able to stand up in it. Little or no surface water would escape from the pond
through streams, and they are often fed by underground springs.
Lakes are larger bodies of freshwater formed by some natural process like tectonic plate
movement, landslides, or human actions , such as building a dam. Almost all lakes are
freshwater, and water usually leaves the lake through a river or a stream. All lakes lose some
water to evaporation.
Some lakes are so large that they have their own tidal systems and currents, and can affect
weather patterns. The Great Lakes in the United States, for example, contain 22% of the
world’s fresh surface water (Figure13.12). The largest of the Great Lakes, Lake Superior,
has a tide that rises and falls several centimeters each day. The Great Lakes are large enough
to change the entire weather system in the Northeast region of the United States, in what
is known as the “lake effect.” They are home to countless species of fish and wildlife as well.
Lakes can be formed in a variety of different ways. Some lakes, like The Great Lakes fill
depressions eroded as glaciers scraped soil and rock out from the landscape. Lakes known as
crater lakes, formed in volcanic calderas that have filled up with precipitation. Rift lakes are
formed in cracks created by tectonic faults. And subglacial lakes are found below a frozen
ice cap. As a result of geologic history and the arrangement of land masses on the Earth,
most lakes are in the Northern Hemisphere. In fact, over 60% of all the world’s lakes are in
Canada — most of these lakes were formed by the glaciers that covered most of Canada in