DRAINAGE BASINS
Important Definitions / Notes
 Tributary: a stream or river that joins a larger river.
 Confluence: the point at which two rivers meet.
 Precipitation: any water that falls from the atmosphere
viz. rain snow, sleet or hail.
 Headwaters / Source: the furthest point along the
course of a river from its estuary, mouth or confluence
with another river i.e. where the river starts.
Confluence and tributary of Sligachan
River, Isle of Skye.
Pollution at the mouth of the Millers River
where it enters the Connecticut River.
Millers River is the tributary and its mouth
is also a confluence.
Important Definitions / Notes
 Headwaters may be in a marsh or formed from
glacial melt waters, in a lake or spring.
 Mouth: the part of the river that flows into a lake,
reservoir, sea, ocean or estuary.
 Groundwater:
water stored underground
permeable rock such chalk or sandstone.
in
The source of Nile from the
underwater spring at the neck of
Lake Victoria, Jinja.
The source of the Irrawaddy
River – Lake Laka
The source of the Rhume River
in Germany is a karst spring
Source of the River Ethiope,
Nigeria
–
deepest
inland
waterway in Africa – is a spring
at the base of a silk cotton tree
El Nevado Mismi: This ice
mountain is on the continental
divide, one side drains through
the Amazon River to the Atlantic
Ocean, and the other to the
Pacific Ocean.
The source of the River Severn
– the longest river in England is a deep, blanket-peat bog in
Wales.
Mouth of Cass River in
Lake Tekapo
Mouth of Hoh River
Features of a
Drainage Basin
What is a drainage basin?
1st Definition
2nd definition
 An extant or area of land
 The area of land drained by
within which surface and
groundwater
from
rain,
melting snow or ice is
transferred to a single point
at lower elevation, usually
the exit of the basin, where
the waters join another
water body, such as a river,
lake,
reservoir,
estuary,
wetland, sea or ocean.
a river system i.e. a river
and its tributaries.
 It includes water found in the
water table as well as
surface runoff.
What is a drainage basin?
 Drainage basins drain into other drainage basins in a
hierarchical pattern, with smaller sub-drainage
basins
or sub-catchments combining into larger
drainage basins.
 The drainage basin acts as a funnel, collecting all the
water within the area covered by the basin and
channeling it to a single point such as the ocean, a lake
or a larger stream.
 Tributaries and the main stem river serve to drain the
surrounding drainage basin of its surface water
and groundwater by leading the water out into an ocean,
sea, lake, estuary, reservoir or another river.
 Each
drainage basin /sub-basin is separated
topographically by a perimeter called a drainage divide.
An example of a
drainage basin.
All water collected
within the area
enclosed by the
dashed line is
channeled to a
single point.
The dashed line is
the main water
divide or drainage
divide.
Main and sub-basins
What is a drainage basin?
 In a drainage basin the tributary stream (A) of a
brook (B) that joins a small river (C) is the tributary of
a larger river (A is tributary of B, which is larger than
A. B is a tributary of C, which is larger than B and A)
 For water to flow from A to B, the drainage basin of A
has to be at a higher elevation than B. For the same
reason, B is at a higher elevation than C.
 A, B and C are thus a part of a series of successively
smaller area but higher elevation drainage basins.
 Similarly, the Missouri River is part of its own
drainage basin and that of the Mississippi River.
What is a drainage basin?
 Other terms that are
 In North America, the
used
to
describe
drainage basins are:
term
watershed
is
commonly
used
to
mean a drainage basin,
though in other Englishspeaking countries, it is
used only in its original
sense i.e. to mean
drainage divide
 catchment
 catchment area
 catchment basin,
 drainage area,
 river basin
 water basin.
What is a drainage basin?
 Drainage basins are the
main unit used in the
study of rivers and the
movement of water within
the hydrological cycle.
 This is because the
majority
of
water
that discharges from the
basin outlet originated
as precipitation falling on
the basin.
 Drainage
basins
are
separated
from each
other by watersheds or
drainage divides.
 These are the boundaries
between drainage basins:
all the precipitation on
opposite sides of a
drainage divide will flow
into different drainage
basins.
What is a drainage basin?
 Endorheic Basins: these
 Water will converge to a
are
closed
drainage
basins.
 Areas of inland drainage
that allow no outflow to
external bodies of water
like oceans or seas.
 Water only leaves by
evaporation or seepage.
single point within the
basin called a sink.
 A sink
may be a
permanent lake, seasonal
lake, swamp or an area
where water is lost
underground.
 The bottom of such a
basin is usually occupied
by a salt lake or salt pan.
Endorheic
basin
showing
waterflow input into Üüreg Lake,
western Mongolia.
The Caspian Sea is
the world’s largest
lake. A large part of
Eastern
Europe,
drained by the Volga
River, is part if its
drainage basin.
Okovango
Delta,
Botswana.
Dead Sea and Sea of Galilee,
Jordan – Israel Border.
Watershed
 An area or ridge of land
 In hilly areas, the divide
that separates waters
flowing
to
different
rivers, basins, or seas.
 A watershed is an
imaginary
line
separating
adjacent
basins.
 Streams on either side
of a watershed will flow
in opposite directions.
lies along ridges and
may be in the form of a
single line of hills or
mountains known as a
dividing range.
 In
flat areas, the
location of the divide is
harder to discern.
Watershed
 Continental
Divide:
where waters on each
side flow to different
oceans e.g. the CongoNile Divide.
 Major Drainage Divide:
waters on both sides do
not meet but flow into the
same ocean e.g. the
Yellow and Yangtze River
Divide
 Minor Drainage Divide:
waters part but eventually
rejoin e.g. the Mississippi
and Missouri River Divide.
The position of a watershed is
usually shown on maps/diagrams
as a dotted/dashed line.
In reality, the watershed will take
the form of a ridge. Thus
precipitation falling on such a
ridge will flow (above or below
ground) in opposite directions.
Diagram of a drainage basin
showing the watersheds or
divides.
Global Hydrological Cycle
 This is the name given to the continuous movement
of water between the atmosphere, lithosphere and
biosphere.
 The total mass/volume of water remains constant
over time.
 The partitioning of water into the major reservoirs of
ice, fresh water, saline water and atmospheric water
is variable and dependent on a wide range of
climatic variables.
 Thus, it is a closed system, with no inputs and
outputs or losses. Instead there are transfers
between stores.
Global Hydrological Cycle
 Water moves from one reservoir to another e.g. river






to ocean or ocean to atmosphere, by the physical
processes of:
Evaporation
Condensation
Precipitation
Infiltration
Runoff
Sub-surface flow
Basin Hydrological Cycle
 In both hydrology and fluvial geomorphology, the
drainage basin is the main unit of focus for study.
 Thus, when studying rivers, frequent mention is
made of the basin’s hydrological cycle.
 When doing this, the drainage basin is said to be the
unit of study as opposed to the global system.
 A drainage basin is a local open system, with
inputs, outputs and processes/transfers.
Basin Hydrological Cycle
 At the local scale the cycle has a single major input –
precipitation (PPT).
 There are two major losses or outputs –
evapotranspiration (EVT) and runoff.
 Water is stored at a number of places within the
system – vegetation, surface, soil moisture,
groundwater and water channels.
The basin hydrological cycle is
also an open system!
Inputs
Outputs
 Precipitation (including rain
 Evapotranspiration
and snow)
 Solar energy for evaporation
and transpiration.
vegetation
 Run-off into the sea
 Percolation of water to
underlying rock strata into
underground stores
from
The basin hydrological cycle is
also an open system!
Stores
 Surface
storage such as
puddles, rivers and lakes
 Glaciers
 Soil storage and ground
water storage
 Water stored on vegetation
(interception)
following
precipitation.
Transfers/Flows
 Percolation
 Overland flow
 Infiltration
 Stemflow
 Throughflow
Human modification of the
system
 Large-scale changes of
channel flow
 Irrigation
 Drainage
 Abstraction
of
groundwater
and
surface
water
for
domestic and industrial
use