river landform

Rivers: Profiles & Landforms

Pankaj Kumar Singh

River ProfileSourceof river

UpperCourse

MiddleCourse

LowerCourse

Cross sectional Profile – Width of RiverLongitudinal Profile - From Source to mouth

( Length of River )

River system

Headwater

TributariesTrunk stream

Distributaries

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 sidesZig-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.


Waterfall formation

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


Waterfall formation

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


PLUNGE POOLUNDERCUTTING

OF SOFT ROCK

OVERHANG

WATERFALL RETREATS . .UPSTREAM . .

Formation of rapids

Resistant rock Less Resistant Rock

Middle Course - Channel 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.

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

• Turbulance 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

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

Gentle valley sides

Flat floodplain

Layers of siltDeposited during floods

Coarse materialForms naturallevees

River is actually flowing aboveThe floodplain !!

Levees is oftenartificiallystrengthened

Levees

Oxbow lakes

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

Oxbow lakes

NARROW MEANDER NECK

FUTUREOX-BOW LAKE

river landform

Environment