Rivers: Profiles & Landforms

Higher Geography

The Hydrosphere

River ProfileSourceof river

UpperCourse

MiddleCourse

LowerCourse

River speed

Fastest??

Fastest??

Fastest??

Reality Landscapes have been modified by ice. Waterfalls and rapids interrupt smooth profiles.

Upper Course

Lower Course

Contrasting river landforms from source to mouth

Channel features

Valley features

Long profile

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.

Vertical erosion

Pothole formation

Upper Course - Valley features

• Valley sides are steep and form a ‘V’ shaped cross section.

• Interlocking spurs.

V shaped valley

Steep sides

Zig-zag bends

(interlocking spurs)

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.

Interlockingspurs

River flows aroundinterlocking spurs

Upper Course – Long Profile

• Generally the gradient is steep and the profile is uneven, particularly where waterfalls and rapids form.

Waterfall formation

Hard Rock – LavaSoft Rock – Sandstone or Conglomerates

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

So over time a ledge develops.

Waterfall formation

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

Hard Rock – LavaSoft Rock – Sandstone or Conglomerates

Waterfall formation

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

Hard Rock – LavaSoft Rock – Sandstone or Conglomerates

Waterfall formation

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

Hard Rock – LavaSoft Rock – Sandstone or Conglomerates

PLUNGE POOLUNDERCUTTIN

G OF SOFT ROCK

OVERHANG

WATERFALL RETREATS . .UPSTREAM . .

Formation of rapids

Resistant rock

Less Resistant Rock

Middle Course - Channel features

• Channel is now wider and has smoother banks and bed compared to the upper course.

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.

Middle Course 1

Middle Course 2

MeandersA meander starts as a slight bend:

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 (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

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.

MEANDERS

Flood plain

MeandersMost erosionon the outsideof the bend .. Fastest flow

Possible breakthrough point

Possible ox-bowlake

Meanders

Middle Course - Profile

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

• 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

• 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

Lower Course

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.

Gentle valley sides

Flat floodplain

Layers of siltDeposited during floods

Coarse materialForms naturallevees

River is actually flowing aboveThe floodplain !!

Levees is oftenartificiallystrengthened

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.

Braided channels

Braided channels

Ox-bow lakesDownstream migration of meanders produce pronounced meander loops whichmay 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

NARROW MEANDER NECK

FUTUREOX-BOW LAKE

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.

Team work and Alloa

tributaries

ox-bowlake

meander

V shaped valley

SteepValleysides

Floodplain

Rivers on OS Maps