Geoscience Reference
In-Depth Information
Overland Flow and
Drainage Basins
source of stream water is water that seeps out of the ground
where stream channels intersect the water table, as was briefly
discussed in Chapter 15 (see Figure 15.3). In some areas, such
as high mountain areas, stream water comes from melting snow
and glaciers.
Still another important source for streams is water that is
stored in lakes and ponds that are fed by both precipitation and
groundwater. If the water level in such a lake reaches a sufficient
depth, it may spill over at a low spot on the landscape, called
an
outlet
, and become the source of a river. Many rivers start
at such an outlet. An excellent example is Lake Victoria in east
central Africa, which is the source of the Nile River. The lake
straddles the borders of Uganda, Kenya, and Tanzania (Figure
16.2a) and lies in a depression created by the East Africa Rift
system (Figure 13.9). Approximately 67,850 km
2
(26,197 mi
2
)
in size, Lake Victoria is the second largest freshwater lake in
area on Earth (behind Lake Superior in North America). Ex-
cess water spills over an outlet at the north end of Lake Victoria
(Figure 16.2b) and marks the beginning of the Nile River. The
Nile River is about 10,730 km (6670 mi) long, making it the
longest river in the world, and flows northward through Sudan
and Egypt to the Mediterranean Sea. The best-known river in
North America, the Mississippi, also begins at a lake outlet,
Lake Itasca in Minnesota (Figure 16.2c). At this place, you can
literally walk across the Mississippi River!
Another significant source of stream water is surface
runoff. Runoff occurs most commonly in association with wet
periods when soils are saturated and pore spaces can no lon-
ger absorb additional precipitation. As discussed in Chapter 15,
these periods are times of groundwater recharge because water
Streams are important features for a variety of reasons. In the
context of the hydrologic cycle, they connect the interior of
continents with oceans. Water that falls as precipitation in high
mountainous areas, for example, will ultimately flow back to
the sea. Flowing water is also the most powerful geomorphic
agent on Earth, shaping the landscape in both subtle and pro-
found ways. Streams are important to humans because they are
sources of water for both direct consumption and agriculture. In
addition, people have long used streams as transportation net-
works to move economic goods from one place to another. As a
result, rivers such as the Rhine in Europe, the Nile in Africa, the
Mekong in Southeast Asia, the Amazon in South America, and
the Hudson and Mississippi in the United States have played
critical roles in the evolution of human societies. Figure 16.1
shows the location of major rivers around the world.
Origin of Streams
A good place to begin considering streams is their origin and
the sources of water within them. Some streams flow all year
long and are called
perennial streams
, whereas others contain
water for only a short time and are thus
ephemeral (
or
intermit-
tent) streams
. Although you might think stream water comes
directly from precipitation that falls into the channel, it is only
a negligible contribution. In most places, a more important
Arctic Ocean
Mackenzie
Peace
Rhine
Danube
Volga
Lena
Ob
St. Lawrence
Missouri
Thames
Seine
Rhône
Irtysh
Fraser
Columbia
Colorado
Rio Grande
Ohio
Irrawaddy
Hudson
Tigris
Hw
a
ng-Ho
Indus
Mississippi
Yangtze
Euphrates
Pacific
Ocean
Nile
Senegal
Ganges
Mekong
Orinoco
Atlantic
Ocean
Pacific Ocean
Niger
Amazon
Congo
Indian Ocean
Parana
Zambezi
Murray-
Darling
Figure 16.1 Important rivers on Earth.
Rivers are channelized bodies of flowing water that connect the continen-
