Geoscience Reference
In-Depth Information
Rivers shape the Earth's continental landscapes in three main ways: by the erosion, trans-
port, and deposition of sediments. These three processes have been used to recognize a
simple three-part classification of individual rivers and river networks according to the
dominant process in each of three areas: source, transfer, and depositional zones.
The first zone consists of the river's upper reaches, the area from which most of the water
and sediment are derived. This is where most of the river's erosion occurs, and this eroded
material is transported through the second zone to be deposited in the third zone. These
three zones are idealized because some sediment is eroded, stored, and transported in each
of them, but within each zone one process is dominant.
The changes in a river's slope that occur between its upper and lower reaches are reflected
in a graphical measurement known as the 'long profile'. This is a section through the chan-
nel from its headwater to its mouth and is typically concave in shape because the head-
waters are steep and slope decreases progressively in a downstream direction. This gen-
erally smooth, concave-upwards form is sometimes interrupted by outcrops of hard rocks
that produce locally steeper slopes. Rapids form in these areas and the velocity of the river
increases, promoting greater erosion, which over a long time period wears down the ob-
struction. In a place where relatively soft rocks are overlain by much more resistant rocks,
a waterfall may occur. The world's highest waterfall, Angel Falls, or Kerepakupai Merú,
cascades over a very hard sandstone rockface in Venezuela and is an awe-inspiring 979
metres in height.
All of the sediment carried by a river ultimately comes from the erosion of surrounding
slopes and water flowing across and through the land surface, but the immediate supply
comes from the bed and banks of the river channel. The flow of water carries this sediment
in three ways: dissolved material - such as calcium, magnesium, and other minerals -
moves in solution; small particles are carried in suspension; and larger particles are trans-
ported along the stream bed by rolling, sliding, or a bouncing movement known as 'sal-
tation'. This material is deposited when circumstances change in some way, such as the
slope of the river bed decreasing, so reducing the river's energy and ability to carry its load.
Much of it is deposited in the sea. Globally, it is estimated that rivers transport around 15
billion tonnes of suspended material annually to the oceans, plus about another 4 billion
tonnes of dissolved material.
In its upper reaches, a river might flow across bedrock but further downstream this is much
less likely. Alluvial rivers are flanked by a floodplain, the channel cut into material that the
river itself has transported and deposited. The floodplain is a relatively flat area which is
periodically inundated during periods of high flow, typically every one or two years. When
