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How rivers change from source to mouth
Rivers change immensely on their journey from Source areas (where they start) to their finishing point at their mouths. There are 4 major concepts here;
The drainage basin
The water cycle
The changing characteristics of rivers from the mountains
The changing valley
shape
Tasks
The drainage basin
This is the area of land drained by a river system (a
river and its tributaries). It includes the surface run-off in the
water cycle, as well as
the
water table. Drainage
basins are separated by
watersheds. A drainage
basin is an example of an open system because
it is open to inputs from outside, such as precipitation, and is responsible for
outputs out of the system, such as output of water into the sea and
evaporation of water into
the atmosphere. You can see a diagram of the drainage basin below.
The water cycle
This is a series of
processes by which water is evaporated from the sea and eventually condenses
and precipitates over the land. It is the main input to the drainage basin
and you can read detailed notes on this process at the
Met Office's website.
Velocity increases as more water is added to rivers via tributary rivers.
This means that less of the water is in contact with the bed of the river
and the mouth so there is less energy used to overcome friction.
Hence rivers flow progressively faster on their journey downstream.
Width and depth increases as more water is added from tributaries.
Gradient (the slope of the land) decreases as rivers flow because the river
meanders across the land rather than erode into it and follow a straight path as
it does in the source. This means
it covers a decrease in height over a longer distance the further downstream you
get.
Stone size decreases downstream and the stones get rounder and smoother as
rivers erode the rocks progressively as the stones are transported downstream.
The diagram on below shows the major changes downstream.
In the source area the drainage basin (an area of land drained by a river and
its tributaries) contains V shaped valleys and waterfalls, and the dominant
processes are erosion. Erosion
tends to be
vertical (straight down into the land).
In the middle section of the drainage basin the river starts to
erode laterally.
This section contains meanders and Ox bow lakes, and the river creates a
flood plain often with Levees.
Here, Material is deposited and erosion can also occur.
In the lower drainage basin deposition dominates as a river enters a sea or
lake, the valley is at its widest and deltas and estuaries are major landforms
and habitats.