978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

figure 1.26 An entrenched meander on the San Juan
tributary of the Colorado River, USA.
Effect on fluvial features
In situations where a meandering river has been influenced by a change in base
level then entrenched meanders or incised meanders may form. The distinction
between the two forms relates to the speed of erosion. Incised meanders are
asymmetrical in shape as they are eroded more slowly. As the river channel
erodes vertically as well as laterally it will start to undercut on the outside of the
bend creating an overhang in the river cliff. The inside of the bend, due to the
continued deposition, will take the form of a gentle sloping bar.
Entrenched meanders are formed, geologically, more rapidly. As a result
the meanders tend to take a more symmetrical shape as they carve out a deep
winding gorge across the landscape such as the Grand Canyon. Entrenched and
incised meanders are more visual where they have cut through different layers
of bedrock. Gooseneck on the San Juan river, a major tributary of the Colorado
River, is a well known example of an entrenched meander heavily influenced by
the distorted uplift (or upwarp) of the Monument Plateau.
River terraces are areas of higher ground surrounding a river. They are the
former floodplains of the river that were carved out when it was higher up,
which are now above the current levels of flooding. Due to a change in base
level an increase in vertical erosion creates a newly cut river.
Fluvial features: deposition
Deposition of sediment occurs when there is a decrease in energy or an increase
in capacity that makes the river less competent to carry its load. Deposition can
occur at any stage along the river but it is most common in the lower reaches.
figure 1.27 The river terraces of the River Dovey,
Wales, UK.
Floodplains
Floodplains are large areas of flat land surrounding a river channel. They are the
areas most susceptible to flooding. Initially cut by a river, a floodplain is made
up of a large amount of alluvial deposits (silt) dropped during times of flood.
As a result they are often fertile and used extensively for agriculture. As the river
spills over the floodplain in times of flood, there is an increase in friction, a loss
of energy and resultant deposition of material. Repeated flooding causes the
deposits to build up in height forming a series of layers high above the bedrock.
The edge of the floodplain is marked by a slightly raised line known as a bluff.
Levees
When a river floods its banks the coarsest material is often deposited first
creating a ridge along the edge of the river channel. Over time more sediments
may be added to the ridge thus creating a natural preventative barrier to
flooding. In low lying areas such as in Holland and New Orleans artificial levees
have been built in response to the threat of flooding.
figure 1.28 Braiding on the White River, Washington, USA.
figure 1.29 The Nile Delta, Egypt, flowing into the
Mediterranean Sea.
28
Hydrology and fluvial geomorphology
Braiding
Braiding occurs when there is a high proportion of load in relation to the
discharge. This may be the result of seasonal changes and snow-melt, such as in
the Alps. At times of low flow the river may be forced to cut a series of paths that
converge and diverge as they weave through large expanses of deposited material.
Braiding begins with a mid-channel bar that grows downstream as the
discharge decreases following a flood. The coarse bedload is deposited first. This
forms the basis of bars and, as the flood is reduced, finer sediment is deposited.
The upstream end becomes stabilised and over time can become vegetated. These
islands can alter subsequent flows, diverting the river and increasing friction.
Deltas
Deltas are formed when large amounts of river load meet the sea and are
deposited. Deltas are usually composed of fine sediments that are dropped
during low energy conditions and are so called because they are triangular in
shape, which is similar to the shape of ‘delta’, the fourth letter of the Greek
alphabet. As freshwater and saltwater mix, clay particles coagulate (stick
together) and settle to the seabed in a process known as flocculation.
The finest sediments are carried furthest and are the first to be deposited as
bottomset beds. Slightly coarser material is transported less far and deposited
as foreset beds, while the coarsest material is deposited as topset beds.