From Source to Sea
From the mountain top to the sea, a river constantly changes the
surface of the land. On its journey it destroys rocks, moves boulders,
and deposits sediments, constantly wearing away all of the land
in its path
The Upper Reaches of a River
Rivers begin their life in hills or mountains, where rainwater
collects and percolates into the soil becomes groundwater. It trickles
through spaces in sand or gravel or between fractures in rocks to
discharge into a stream forms tiny streams known as rivulets or
gullies. As the land becomes saturated, excess rain flows from the
land as surface runoff.
The start of a river is called its source. These tiny rivulets
grow larger and collect more water and become streams themselves
or flow into streams and add to the water already in the stream.
One of the groundwater streams that mark
the start of the River Exe
The Middle Reaches of a River
Once the volume of water in the stream increases the water carries
material along and wearing away the land surface. By carving and
moving rocks (sediment), running water has tremendous power to change
the earth's surface. The areas in which the streams run form depressions
between hills or mountains are known as valleys. They are usually
deep, steep V-shaped valley as the fast moving water cuts away at
the rock as it flows downhill.
Typical V-shaped valley on Exmoor
The Lower Reaches of a River
When the river leaves the higher ground and enters the flat plains,
the river slows down. As the force of the water is reduced the sediment
falls to the river bottom and is dumped on the riverbed. Most of
the sediment deposition occurs in the river flood plains. On the
flat plain the river flows slowly, making S-shaped curves known
as meanders. When the river floods, the river will spread out over
many miles on either side of its banks.
At the end of its life, the river flows into a large body of water,
such as an ocean, bay, or occasionally a lake to end its journey.
At this final stage the large amount of sediment can be deposited
to form sand banks and the river water has to find its way to the
sea in the multi-channels that make up a delta or estuary
The complete river pattern that results is like a tree-structure
and is known as the river drainage basin or catchment area.
 link
to Stream Order
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