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Rivers: Profiles & Landforms. Higher Geography The Hydrosphere. Source. Upper. Middle. Lower. of river. Course. Course. Course. River Profile. River speed. Fastest? ?. Fastest? ?. Fastest? ?. Landscapes have been modified by ice. Waterfalls and rapids interrupt smooth profiles.

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rivers profiles landforms

Rivers: Profiles & Landforms

Higher Geography

The Hydrosphere

river profile

Source

Upper

Middle

Lower

of river

Course

Course

Course

River Profile
river speed
River speed

Fastest??

Fastest??

Fastest??

reality

Landscapes have been modified by ice.

  • Waterfalls and rapids interrupt smooth profiles.
Reality
slide5

Upper Course

Lower Course

contrasting river landforms from source to mouth

Contrasting river landforms from source to mouth

Channel features

Valley features

Long profile

upper course channel features
Upper Course - Channel features
  • River channel is rocky.
  • Covered with various shapes and sizes of boulder.
  • Discharge is low.
  • Under flood conditions rivers energy is expended on vertical erosion with hydraulic action and corrosion processes at work.
  • Potholes may form.
upper course valley features

V shaped valley

Zig-zag bends

(interlocking spurs)

Steep sides

Upper Course - Valley features
  • Valley sides are steep and form a ‘V’ shaped cross section.
  • Interlocking spurs.
v shaped valley
V – shaped valley

Form due to a combination of the following processes:

Vertical erosion by the river itself.

Physical weathering (eg: frost action) which provides debris to move down slope.

Mass movement (inc: soil creep & landslides) to move debris down slope.

interlocking spurs
Interlockingspurs

River flows around

interlocking spurs

upper course long profile
Upper Course – Long Profile
  • Generally the gradient is steep and the profile is uneven, particularly where waterfalls and rapids form.
waterfall formation
Waterfall formation

Soft rock is easy to erode, but the hard rock is resistant.

So over time a ledge develops.

Example

Hard Rock – Lava

Soft Rock – Sandstone or Conglomerates

waterfall formation1
Waterfall formation

The water rushes over the ledge and erodes a plunge pool by abrasion and hydraulic action.

Example

Hard Rock – Lava

Soft Rock – Sandstone or Conglomerates

waterfall formation2
Waterfall formation

The ledge collapses into the plunge pool, where the debris helps to speed up the erosion.

Example

Hard Rock – Lava

Soft Rock – Sandstone or Conglomerates

waterfall formation3
Waterfall formation

The process is repeated and the waterfall gradually retreats upstream, carving out a gorge.

Example

Hard Rock – Lava

Soft Rock – Sandstone or Conglomerates

slide19

WATERFALL

RETREATS . .

OVERHANG

PLUNGE

POOL

UNDERCUTTING

OF SOFT ROCK

UPSTREAM . .

middle course channel features
Middle Course - Channel features
  • Channel is now wider and has smoother banks and bed compared to the upper course.
middle course valley features
Middle Course - Valley features
  • River erosional energy is now increasingly expended horizontally rather than vertically.
  • Lateral erosion by the river’s meanders broadens the valley floor into a narrow flood plain.
  • Meanders gradually shift their course downstream.
meanders

Water flows faster on the outer curve of the bend (more energy), and slowest on the inner curve (less energy).

So the outer bank gets eroded while material is deposited at the inner bank.

Over time the outer bank gets worn away (river cliff) and the inner one builds up (river beach). The bend grows into a meander.

Meanders

A meander starts as a slight bend:

meanders refer to previous notes and diagrams
Meanders (refer to previous notes and diagrams)
  • Alternating series of irregularities develop
  • Pools – deeper stretches of slow moving water
  • Riffles – shallower section of faster flow, flowing above coarser material
  • River develops a winding or sinuous course
  • Faster flow on outer bend results in erosion and formation of River Cliff
  • Slower flow on inside of bend results in deposition and formation of Slip-off Slope
meanders1
Meanders
  • Meanders develop and migrate laterally and downstream
  • Helicoidal flow further assists meander formation and transports sediment from river cliff to the slip-off slope on the inside of the next bend.
slide28

MEANDERS

Most erosion

on the outside

of the bend .. Fastest flow

Flood plain

Possible break

through point

Meanders

Possible

ox-bow

lake

middle course profile1
Middle Course - Profile
  • Gradient is now less steep.
  • Overall the profile is smooth – but an outcrop of resistant rock could still cause a waterfall.
lower course channel features
Lower Course - Channel features
  • The channel is now at its broadest and deepest.
  • Bedload is carried entirely in suspension and is solution.
  • Deposition now dominates – particularly during floods.
  • Erosion also occurs – in the formation of meanders
lower course valley features
Lower Course – Valley features
  • Thanks to lateral erosion the valley sides may now be several kilometres away.
  • Typically it may also contain the following features:
    • Floodplain & natural levées
    • Braided channels
    • Meanders
    • Oxbow lakes
    • Estuaries and deltas
natural lev es
Natural levées
  • As the river floods, sediment is dropped over all the flooded areas but most falls along the river channel itself.
  • This sediment raises the height of the banks is flooding occurs regularly
  • Levees themselves do not prevent flooding because as the banks are raised, more sediment is dropped on the river bed, raising the water level.
slide36

River is actually flowing above

The floodplain !!

Gentle valley sides

Levees is often

artificially

strengthened

Flat floodplain

Coarse material

Forms natural

levees

Layers of silt

Deposited during floods

braided channels
Braided channels
  • Formed by the choking of the main channel by the deposition of a considerable amounts of the river load.
  • The channel splits into several smaller channels which flow around fresh ‘islands’ of deposited material before rejoining.
ox bow lakes
Ox-bow lakes

Downstream migration of meanders produce pronounced meander loops which

may form ox-bow lakes during flood conditions

1 Lateral erosion on outside of bends of pronounced meander

2 Narrow neck of meander gradually becomes narrower

3 Neck is cut through by river during floods and river forms new straighter channel

4 Cut-off is sealed by deposition

5 Ox-bow lake begins to silt up

(Core Higher, P 65, Fig 3.36)

oxbow lakes

An Oxbow lake starts as a meander. During a flood the river cuts across the meander, forming a new channel.

Sediment is deposited along the sides of the new channel. The loops gets sealed off and an oxbow lake forms.

The water in it becomes stagnant. The lake will remain sealed of until either the river floods into it or it dries out.

Oxbow lakes
slide44

NARROW MEANDER NECK

FUTURE

OX-BOW LAKE

estuaries and deltas
Estuaries and deltas
  • An estuary is where a river meets the sea (it is tidal).
  • Deltas are essentially the seaward extension of the floodplain and form where tides are too weak to remove deposit sediment.
slide47

Rivers on OS Maps

tributaries

V shaped valley

Steep

Valley

sides

meander

ox-bow

lake

Flood

plain