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3. Water on the Earth
3. CONTINENTAL WATERS
Continental waters
are almost exclusively formed by fresh water. As we know,
fresh water constitutes only 3% of all the water on our planet. This water is found
under the ground (called underground water) and on the surface (glaciers, lakes
and rivers).
3.1. Rivers
The
flow rate
is the quantity of water that flows in a river at a specific point in
its course. The flow rate increases during rainy seasons and decreases during dry
seasons. The
river channel
is the space the water in the river occupies as it flows
along its course.
The
river basin
is the area drained by a river and all its tributaries and underground
springs that flow into it. Small rivers have small basins and low flow rates; large
rivers can have basins of millions of square kilometres, and so have high flow
rates.
The course of a river has three parts: upper, middle and lower course.
COURSE OF A RIVER
MOST IMPORTANT RIVERS
IN THE WORLD
River
(continent)
Longitude
(km)
Flow rate
(m
3
/s)
Amazon
(South
America)
7 062 230 000
Nile
(Africa)
6 756 2 800
Chang
Jiang (Asia)
6 300 32 000
Murray-
Darling
(Oceania)
3 672
760
Volga
(Europe)
3 690 8 000
Mississippi-
Missouri
(North
America)
5 971 18 000
In Spain, the Ebro is 930 km long
and has a flow rate of 600 m
3
/s.
Upper course
This begins at the source of the river, which is usually in mountainous areas.
Here, the river has little water or flow rate, but it flows down rapidly due to
the steep inclines and usually flows through narrow spaces. The main effect
of the river is erosion: the wearing away of the rocks in its course.
Middle course
Here, the river leaves the mountains. The
flow rate increases as it receives the water
from its
tributaries
(fluvial system). The
river
transports
materials eroded in the
upper course and deposits the bigger
materials (gravel and pebbles). In this area,
there are towns, villages and crops.
Lower course
This is the last part of the river, where it flows into the sea or ocean at the
river mouth
. Here, the river has more water and so the channel is wider.
Because the land is quite flat, the water moves slowly and deposits smaller
materials (sand, mud and clay) that are good for agriculture. It is common
for ports and major cities to be located at river mouths.
Rivers
are continuous currents of water. They don’t disappear during dry seasons,
such as sometimes occurs in streams. Rivers that flow into other rivers are called
tributaries.
