river channel is the trend in which the river flows. it’s defined by
the river bed and banks.
water flows downhill through the river channel; the water has mass
and velocity, so it has energy to be able to change the shape and
nature of the channel.
considerable changes happens as the water flows from the source
to mouth.
changes shown by a long profile
Bradshaw’s model shows that the river can respond to changes in its inputs of
discharge and sediment by changing any one of the variable
Erosion
Erosion: wearing away of the surface of the earth.
active process involving movement. rivers erode channels as they flow downhill
towards the sea. the river energy erodes the channel in five ways;
abrasion (corrasion): river uses its load of sediment to wear away its bed and
banks.
Attrition: particles of sediment in the load of the river bump into each other and
wear each other away. the result is sediment becoming smaller and more rounded
as its carried downstream.
hydraulic action: direct force of the flowing water can break material from the bed
and banks.
Cavitation: force of exploding air.
solution (corrosion): natural river water is slightly
acidic and can dissolve rocks such as chalk and
limestone.
Transportation
river transports the load that’s supplied to them in four main ways…
traction: larger particles of bedload are rolled along by the force of flowing water.
saltation: smaller bedload particles hop along the river bed.
suspension: accounts for most of the load, especially in lowlands. lowland rivers
look muddy and brown as there’s large amounts of sand, silt and clay suspended in
the water.
solution: the dissolved load in derived from soluble rock such as limestone and
chalk. chalk streams are often clear as the dissolved load isn’t visible.
the load varies as the energy changes. at high discharge, large amounts of
sediment can be carried.
load is calculated at the bankfull stage (point when the river is flowing most
efficiently, just before it spills onto its floodplain).
Capacity: the total amount of load that it’s carrying.
Competence: maximum size of particle that the river is capable of transporting at
the bankfull stage.
Deposition and sedimentation
when a river slows, they have less energy and deposition takes place when a river
loses energy. larger particles are deposited first. river energy depends on its
velocity and discharge. load can be dropped as the velocity has slowed or the
discharge has fallen.
deposition happens in the following circumstances: during a period of low
discharge when there has been a dry spell with no rain, on the inside of a meander
bend, when a river bursts its banks due to a reduction in the hydraulic radius, when
the load is increased (after deforestation), when a river enters the still water of a
lake or sea
sedimentation occurs when river sediment is deposited from still water, common
process on floodplains and sea bed. on the sea bed it’s aided by the process of
flocculation, the way that charged ions in sea water allow clay particles to clot
together and settle out of suspension. the bottom set, fore set and top set beds in
a delta are produced by this. material deposited as sediments may become
sedimentary rock, links river process to rock cycle.
Hjulstrom curve
erosion velocity/entrainment velocity: river velocity
required to pick up particles of different sizes.
fall velocity/settling velocity: speed the river has to slow
to, before particles will be deposited.
shows relationship between particle size and velocity.
uses logarithmic scales for y and x axis. known as a log/log graph = allows a wide
range of data to be shown on a small graph.
velocity needed to keep particles moving is LOWER than velocity needed to start
them moving. if the river starts to move a particle faster, the water needs to slow
down before the particle is dropped.
sand is the easiest material to erode, can also be picked up at lower velocities than
smaller or larger particles. clay is cohesive (sticky) and pebbles are heavy, needs
more energy to be eroded than sand.
fine clays stay in suspension even if water stops moving. reason why lowland rivers
always look muddy.
when a river slows, the coarse material is dropped first, finest is dropped last.
reason why levées form close to river during a flood.
Factors affecting the energy of a river
rivers have kinetic energy as they have mass and velocity.
discharge is affected by precipitation and the characteristics of the drainage basin
system. increases as the river flows downstream as more tributaries being water to
the main river.
velocity is mainly affected by friction and gradient. friction is more important than
gradient. friction is measured by bed roughness and hydraulic action.
bed roughness: a rough channel produces more friction and provides more
resistance to river flow than a smooth channel. roughness is measured by
manning’s N. N = R0.67 x S0.5 / V. N = manning’s N — the roughness coefficient. R =
hydraulic radius. S = channel gradient (as a fraction). V = mean velocity of flow.
higher N, the rougher the bed. small mountain streams: 0.05. lowland rivers: 0.015.
hydraulic radius: measures the river’s efficiency. compares the friction with the
discharge. in an efficient river, water moves relatively easily with minimum
resistance. hydraulic radius = channel cross-sectional area (CSA) / wetted
perimeter (WP). CSA: channel depth x channel width. WP = length of bed and
banks in direct contact with the water. hydraulic radius increases downstream, also
changes as the discharge changes at any one point along a river
Patterns of flow
laminar flow: water flowing downwards over a smooth surface
can flow in a simple sheet. can be seen on paved surface and
it’s rare in nature as most surfaces exert enough friction for
turbulence to disrupt the flowing sheet
turbulent flow: water in river channel is subject to friction with
bed and banks. water closest to bed and bank slows down and
water in the middle overtakes slow water. results in turbulence.
water at sides of river goes towards the banks and water at bottom of river goes
downwards
helicoidal flow: line of fastest flow (thalweg) follows corkscrew/spiralling path as
river moves downstream. related to development of meanders but in straight
channels it can be seen. thalweg is constrained by river channel. spiral from
surface to bed to surface and from side to side in downstream direction. vertical
movement of thalweg produces pools and riffles and bank to bank creates erosion
on each bank. helps form regular spaced meanders along river’s course.
Channel types
sinuosity: measure of how bendy a river channel is (done in sections). length river /
length of valley. under 1.5; straight, over 1.5; meandering.
Straight river channels: rare as helicoidal flow dominates most rivers and makes
them meander. thalweg moves from side to side because helicoidal flow.
Meandering river channels: upland streams meander but most pronounced
meanders are on floodplains where lateral erosion is made easier by the soft
nature of river banks. very common as spiralling is normal behaviour of moving
fluids. rivers are confined to channels so often spiral downwards and produce
helicoidal flow. not produced by large obstacles.
Braided river channels: contain large numbers of islands/bars made from sediment.
found where discharge varies often in the year and where a large amount of sediment is being carried by river. braiding results from deposition of sediment on
riverbed during a time of falling discharge. river splits as flows around deposits.
extremely wide and constantly changing.
Upper course landforms
• Potholes: formed by turbulence which swirls pebbles around in a depression on the
river bed. swirling pebbles enlarge the pothole by the process of abrasion. small
features and evidence that vertical erosion predominates in upland rivers.
• Rapids: common in upper course of the river. form at places where gradient is steep
and river bed is rocky, resistant to erosion and irregular. usually caused by a band
of hard rock in the river bed.
• Waterfalls: form where a horizontal layer of hard rock lies on
top a layer of softer rock in river valley. soft rock underneath
is eroded faster by river and gradually a plunge pool
develops. splashing water and eddy currents in plunge pool
undercut the hard rock layer above. eventually creates
unsupported overhang of hard rock. overhang collapses into
plunge pool. if undercutting and collapse process is repeated
over long period of time, waterfall retreats upstream, forms deep steep sided valley
called a gorge.
• Gorges: form best when the hard rock is especially resistant to weathering but gets
eroded. Form by waterfall retreating. in semi-arid areas with short wet season
leading to vertical erosion of riverbed when river is flowing, but no water for
weathering at other times of the year. where mountain range has formed across
river path, but vertical erosion of river kept up with mountain range growth. known
as antecedent drainage.
Lower course landforms
floodplain is the flat land next to the river which is liable to flood when the river
rises after heavy rainfall. often badly drained, with marshes and ox-bow lakes.
unhealthy areas to live in as diseases are common. sometimes river flows above
level of surrounding floodplain but enclosed by natural embankments called
levées.
lateral erosion predominates on a floodplain. river is close to sea level, base level,
so can’t cut down much further. floodplain is made of soft alluvium (as formed by
river material deposition) so erosion happens easier and meander belt migrants
constantly across the floodplain. where meander reaches floodplain edge, may
erode back the low valley side, helps to maintain the low bluffs found at side of
most floodplains.
Main deposition forms that contribute to floodplain formation: fine silt and mud
(part of suspended load.) as floodwater spreads across floodplain, hydraulic radius
decreases, friction more important, river slows, and deposition happens. deposition
takes place closest to river. areas further from river get thinner sediment layers
and don’t grow up fast. leads to lower areas known as back swamps. point bar
deposition in slow water on inside of meander. deposits spread across floodplain as
meanders migrate. sediment deposition on river bed at times of low water when
velocity slows. why big rivers raise above floodplain level.
levees are depositional features. when rivers reach bankfull stage and burst their
banks, current slows, and deposition happens. biggest
particles dropped first and when river levels fall after flood,
course deposits form embankments are each side of river.
natural features but people often raise and strengthen them to prevent flooding. artificial embankments are built for same purpose, in USA
embankments called levees.
Landforms produced by sedimentation
deltas: depositional features which form when river meets sea or runs into lake. a
large sediment deposition at the end of a river or stream. when river meets still
water, velocity loss leads to energy loss and rivers sediment in dropped. sea water
contains charged ions of the salts dissolved in it and charged ions lead to clay
particles flocculate — tiny particles cluster together, becoming bigger and settle to
bottom. sediment deposition blocks the rivers main channel which splits into
distributaries. continued deposition means delta grows outwards into sea, forms a
flat marshy land extension. lakes and lagoons within delta. as deltas formed of
fertile alluvium, they’re attractive for settlement. dangerous place to live as
susceptible to flooding by river and sea.
Arcuate delta: fan-shaped and form when tidal range is low and strong movement
of sediment in 1 direction along coast. keeps seaward delta edge smooth in shape.
E.g. Niger
Cuspate delta: ship of arrowhead or worn tooth. low tidal range and 2 offshore
currents shape this delta, going in opposite directions at different times of the
year. E.g. Ebro
Birds foot delta: formed in low tidal range and strong river currents. no clear
offshore current to shape delta so each distributary builds land out into sea. E.g.
Mississippi
Landforms produced by erosion and deposition
Meanders: are most typical of all river landforms. found at any point
all river course. used to be thought meanders were caused by
obstacle along river course, causes it to deflect from a straight
course. once initiated, different erosion and deposition rates ensured
that meanders remained and developed. idea was doubted when saw
that were certain regularities and relationships found wherever they
were found. whatever the river size, wavelength of its meanders
were 8-10x width of river.
Pools and riffles: straight rivers develop deeper sections where
erosion predominated (pools) and shallower sections where sediment
has deposited (riffles). complex cause and not understood but regular
spacing of pools and riffles (distance from pool to pool is 4-5x width of river)
suggests related to helicoidal flow. thalweg moves in corkscrew fashion (helicoidal)
and the rising and falling of zone of max velocity with river corresponded to pools
and riffles position.
meanders have an asymmetric cross section and water flow in 3D is complex. form
of meander is same wherever found. deep water and a river cliff found on outside
of bend whereas shallow water and depositional features (slip-off slope or point
bar) found on inside. related to nature of river flow, especially different energy
amounts river has on inside and outside of bend.
Ox bow lake formation: alluvium of floodplain is soft as river close to sea level,
lateral erosion predominates. Erosion focused on outside bend and deposition on
inside. meander becomes more sinuous. continued erosion on the outside of bend
undercuts the river cliff; retreats. deposition of material on the point bar on inside
of bend continues. together these processes move the whole meander sideways.
meander neck becomes narrower as two river cliffs move closer together. Ox bow
lake is formed. during a large flood, when water is moving rapidly, erosion of two cliffs finally removes the neck of land between them. river adopts a more direct
line of low as increases gradient off river bed and makes river flow more efficient.
deposition in the still water of old meander cuts meander off from new course of
river. ox-bow lake is a temp feature as vegetation growth fills it up and turns it into
area of marshy ground — much like rest of natural floodplain.