Higher geography: Hydrosphere
DRAINAGE BASIN
Drainage Basins
Rivers and their tributary stream form a network into which the water of a landscape drains. These river systems
are capable of transporting water and eroded material from land to sea. The area drained by this system is called a
drainage basin (or catchment). The boundary around a drainage basin is called the watershed. This is the
perimeter of high ground separating drainage basins.
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Within a large drainage basin there are ever-diminishing numbers of catchments, each one smaller than the
last. The small streams drain into large streams which drain into larger tributary rivers and eventually into
the main trunk river.
The Amazon is the world’s largest basin. The Tay drainage basin is Britain’s largest which starts up at
Kenmore and loch Tay and eventually meets the sea when the trunk river’s mouth opens up to the North
sea at Dundee.
Annotate the
map of the
Drainage Basin
MEASURING DRAINAGE BASINS:
There are two key ways to measure a drainage basin.
Stream Ordering
Drainage Density
Strahler stream ordering (named after Strahler) analyses the
network of streams. The diagram below shows a l
1. First order streams are the large number of unbranched
tributaries that start at each individual source and run
to the first confluence.
2. When two first order streams meet they become a
second order.
3. Two second orders become a third order, and so on…
4. The drainage basin is ranked according to the highest
order stream it flows into. The Mississippi is huge and
only attains a tenth order status.
Drainage Density = total length of all stream
within the drainage basin (km)
area of the basin (km2)
In Britain, drainage density figures range from 24km of river per km2. The higher density of
rivers s found where precipitation is heavy and
constant, slopes are steep, rocks are
impermeable and vegetation is sparse. This all
increases the flow of water into the river. High
drainage density also means that there is a high
likelihood of flooding.
Higher geography: Hydrosphere
DRAINAGE BASIN
The Drainage Basin as an Open System
Drainage Basins are part of the global hydrological cycle. The larger global hydrological cycle is a closed system
with a fixed amount of water circulating within it. On the other hand, each drainage basin is an open system with
independent inputs and outputs that is transferred through the system and occasionally stored in reservoirs, rocks
and the soil. The following elements in a drainage basin can be identified:
INPUTS: water that falls into the drainage basin
PRECIPITATION
OUTPUTS: when the water is released from the cycle
EVAPOTRANSPIRATION
RIVER MOUTH
STORAGE: water that is stored. For example, lochs, reservoirs and soil
INTERCEPTION
SURFACE STORAGE
SOIL MOISTURE STORAGE
GROUNDWATER
Higher geography: Hydrosphere
DRAINAGE BASIN
TRANSFERS: water that is stored. For example, lochs, reservoirs and soil
INFILTRATION
PERCOLATION
SURFACE RUN-OFF
THROUGH FLOW
GROUNDWATER FLOW
Higher geography: Hydrosphere
DRAINAGE BASIN
INPUTS
EVAPORATRANSPIRATION
INPUTS
STORAGE
OUTPUTS
TRANSFERS
OUTPUTS
EVAPORATION
STORAGE
TRANSFERS
SURFACE
RUN-OFF
INFILTRATION
CHANNEL
STORAGE (eg.loch)
VEGETATION
STORAGE
Variable level
Water table
1. Fill in the blanks
2. Colour code the processes