Freshwater Rivers
and Lakes Notes
1. Natural courses of fresh water
that flow mostly downhill across
the Earths surface are called rivers.
2. Many rivers start life as
mountain springs or are fed by
lakes or glaciers. Pulled by
gravity, the water flows downhill
toward the sea or ocean.
3. Many rivers flow throughout
the year. Others called arroyos in
the Americas and wadis is Africa
and Asia, are dry except when rain
falls.
4. The Nile River in Africa is the
world’s longest river. It flows
4,187 miles from the equator to the
Mediterranean Sea. The Amazon
in South America flows for 4000
miles. It forms the world’s largest
river system and is over 100 miles
at its mouth.
5. A river is part of a larger
drainage system---a pattern of
fresh water that drains an area of
land. The water collects in small
channels (streams or creeks),
which join together to form larger
channels that eventually form a
river.
6. Channels that flow into rivers
are called branches or tributaries.
The Mississippi drainage system
covers 1.2 million square miles,
with tributary rivers from two
Canadian provinces and 32 U.S.
states.
7. A river’s mouth is where the
river flows into a larger body of
water, such as a lake or and ocean.
8. An estuary is where the lower
part of the river’s course meets the
seawater tides, and so it has a
mixture of fresh and salt water.
9. As a river flows toward the sea,
it carries with it sediment (rock
fragments, clay, silt, or fine sand),
call the river’s load. Fine
sediments are carried along in the
water. Coarse sand, pebbles, and
boulders are dragged along the
riverbed. These loads gradually
erode (wear away) the riverbed
rock to form a channel. The speed
of erosion depends on the type of
rock, the volume of water, and its
speed of flow. A swift-flowing
river carries more rocks and
sediment that a slow moving river,
eroding the land more quickly.
10. The river’s speed depends on
the steepness of the slope over
which it flows. Near the beginning,
or headwaters, of a river, the
slopes are usually steep so the
river flows fast and may cut deep
gorges or canyons through the
landscape. Towards the end of its
course, the river flows more
slowly through land with more
gradual slopes.
11. When the speed of a river slows
down, it can no longer carry all of
its load, so it deposits (drops) some
of its sediment. When the river
reaches a plain (flat area), it may
shed larger quantities of sand,
mud, or gravel to form bars. These
bars may form a network of
shallow, shifting channels in the
riverbed. Some rivers drop coarse
sand or gravel in fan-shaped
deposits called alluvial fans.
12. When the river floods, it
deposits sediment close to its
banks. A series of floods may lead
to the formation of high banks
called natural levees. At its mouth,
the river may fan out and drop
large quantities of fine sediment to
form a delta. The world’s largest
delta lies at the mouth of Ganges
River, in northern India and
Bangladesh.
13. Meanders are natural bends or
loops in a rivers course that forms
in flattish land. Swift-flowing
water erodes the outside of the
bends, while slower-moving water
drops sediments on the inside of
the curves. Gradually, this process
enlarges the meanders and moves
them downstream. The river may
break through the neck of the loop
to form a new course that bypass
the meander. The cut-off loop fills
with rainwater to form a pool
called an oxbow lake.
14. Waterfalls occur when a river
tumbles over a shelf or cliff of
hard rock. Gradually, the water
wears away the softer rock below
the hard rock at the base of the
cliff. An overhang of the harder
rock forms that eventually breaks
off, causing the waterfall to slowly
move backward upstream.
15. There are many bodies of
inland water, such as ponds, lakes,
and inland seas. Lakes are larger
than ponds but smaller that inland
seas. Some lakes are natural:
others are artificial.
16. Many lakes contain fresh water,
but others are salty, although not
as salty as inland seas. There are
about 30,000 cubic miles of
freshwater lakes in the world, and
about 25,000 cubic miles of
saltwater lakes.
17. On a map it is easy to see that
there are many more lakes in some
regions than in others. North
America, especially Canada, has
many lakes. South America has
relatively few lakes, despite the
continent’s high rainfall.
18. In terms of area, the largest lake
in the world is Lake Superior on
the border between Canada and the
United States. It has a surface area
of 32,007 square miles. This
freshwater lake is one of a string of
connected lakes called the Great
Lakes. Together, the Great Lakes
make up 94,700 square miles in
area, and 6,000 cubic miles in
volume. Lake Baikal in Russia is
the worlds’ largest single body of
freshwater. It is also the deepest
lake.
19. Glaciers have been important in
the formation of lakes, particularly
in the northern temperate regions.
As glaciers grew, they moved
slowly down valleys and gouged
out huge amounts of rock. Then,
as the ice age finished and the
glaciers retreated, they left behind
the rocky material. As the ice
melted, natural river dams that
blocked the valleys and created
lakes. The lakes are long and thin,
following the contours of the
valleys.
20. River systems can also produce
lakes. As a lowland river flows
across it floodplain, it often forms
meanders. When the river finds a
shorter route, downstream, a
meander may get cut off. If there
is sufficient water flowing into it
from the plain, this cutoff section
can form an oxbow lake. Flooding
can also produce lakes in a flood
plain.
21. When a river is held up on its
way to the ocean by a barrier such
as sand dunes, a lake can develop
very close to the sea. This lake is
called a coastal lagoon. Its water
may be fresh, brackish, or salty.
The levels of water can vary with
the state of the tide or with
changes in the flow of the river
water entering the lagoon.
22. Lakes can also be formed by
changes in the Earth’s surface.
Very large lakes can result from
faults (breaks) in Earth’s crust, as
in rift valleys. There are two great
series of rift valleys in Africa: the
Western and Eastern rifts.
23. Where the land has slipped
downward between faults, a
graben lake is formed. Lake
Tahoe is a good example.
Volcanic activity can result in
lakes. Crater lakes are formed
when an old volcanic crater fills
with water. Example is Crater lake
in Oregon.
24. Salt lakes form where the
evaporation is high and the flow of
water is not enough to carry the
salt away. Great Salt Lake in Utah
is an example.
25. People have long used lakes as
a way of irrigating (watering)
crops an as sources of drinking
water and food. Lakes have
become more vulnerable to
pollution, due to human sewage
and more intense methods of
farming.
26. Taking too much water from a
lake can also create problems. The
level of Lake Baikal is falling
because irrigation systems take
away more water that flows into
the lake.
27. Artificial lakes have been
created to make reservoirs for
drinking water or as a source of
hydroelectric power. Building
dams to flood valleys has also
made it possible to control river
flow downstream and so avoid the
seasonal flooding of lower areas.