River processes and landforms

Channel processes and featuresValley slope processes and

landforms

RIVER STUDIES

Satellite view of river drainage, Middle East

Microscale drainage basin

Upper valley characteristics

“V”shape valley, vertical erosion dominant

Interlocking spurs

Slumping and landslides - very active hillslopes

Interlocking spurs

Terracettes formed by soil creep

Interlocking spurs

Narrow, shallow channel, low velocity and discharge

Interlocking spurs

Narrow, shallow channel, low velocity and discharge

Large bedload derived from upstream and from valley sides

Interlocking spurs, Robinson, Lake District

A typical upper course valley with interlocking spurs, steep valley sides and active slope processes

Choked runnel, N. Pennines

Ephemeral stream or runnel - water present only during high rainfall events

Debris brought downslope towards channel - dropped when water disappears after storm

Ephemeral stream or runnel - water present only during high rainfall events

Debris brought downslope towards channel - dropped when water disappears after storm

Ephemeral stream or runnel- water present only during high rainfall events

Vertical erosion creating steep, bare slopes vulnerable to further erosion - an example of positive feedback in a slope system

River load in upper course

Load is dumped in summer due to the low discharge unable to carry the c________ and c__________ of load at higher flow levels

Load is dumped in summer due to the low discharge unable to carry the capacity and competence of load at higher flow levels

Boulders are large and semi-rounded, due to attrition within the load and abrasion with the stream bed and banks

Rapids in the Upper Tees Valley

Rapids are mini-waterfalls

Protruding bands of more resistant strata create steps over which rapids fall - the river bed is ungraded

Shallow, slow flowing river due to large amount of friction.

Wetted perimeter is large compared to cross sectional area of water - resulting in a low ????????????

Wetted perimeter is large compared to cross sectional area of water - resulting in a low hydraulic radius (low efficiency)

High Force waterfall, R. Tees

Huge step in river bed due to igneous intrusion of dolerite into the limestone.

Huge step in river bed due to igneous intrusion of dolerite into the limestone. Plunge pool where the dolerite wall is undercut, causing rockfalls and recession of the waterfall upstream

Waterfall creates gorge as it recedes upstream by eroding the base and neck

Plunge pool where the dolerite wall is undercut, causing rockfalls and recession of the waterfall upstream

The long profile will be graded over time

Waterfall creates gorge as it recedes upstream by eroding the base and neck

Plunge pool where the dolerite wall is undercut, causing rockfalls and recession of the waterfall upstream

Headward erosion, Offa’s Dyke

This is amazing!!

Headward erosion, Offa’s Dyke

Spring erodes ground over which it flows and lubricates base of cliff, causing slumping and headward erosion

Headward erosion, R. Colorado

River seeps out from spring at base and undermines steep cliff, causing rockfalls and headwall recession

Headward erosion, R. Colorado

Rivers erode in a headward direction, eating back into plateauRiver seeps out from spring at base and undermines steep cliff, causing rockfalls and headwall recession

Potholes in R. Wharfe

Smooth sculpturing of rock by abrasion, showing evidence of water levels during high discharge events

Vertical erosion is dominant

Close-up of potholes

Close-up of potholesCircular potholes due to eddying motion when river energy is high

Close-up of potholesCircular potholes due to eddying motion when river energy is highPotholes will join together by abrasion, and deepen by vertical erosion

Close-up of potholesCircular potholes due to eddying motion when river energy is high

Load is picked up and used to scour or abrade pothole. Load itself becomes rounded by attrition

Potholes will join together by abrasion, and deepen by vertical erosion

Potholes, human scale!!

Middle course, R. Tees

Valley opens out, more gentle slopes, wider valley bottom

Valley opens out, more gentle slopes, wider valley bottomFirst signs of meanders

Floodplain

River channel wider, deeper, greater velocity and discharge

Meander, R. Lavant, Chichester

Floodplain

FloodplainPoint bar deposits on the inner meander bend where there is low energy

Floodplain

Steep bank known as the river bluff or cliff, caused by concentrated erosion due to the

Point bar deposits on the inner meander bend where there is low energy

Floodplain

Steep bank known as the river bluff or cliff, caused by concentrated erosion due to the thalweg

Floodplain

Steep bank known as the river bluff or cliff, caused by concentrated erosion due to the thalweg and

helicoidal flow

Floodplain

Steep bank known as the river bluff or cliff, caused by concentrated erosion due to the thalweg and

helicoidal flow

riffle

Pools develop at meander bends and riffles in the stretches between bends

Meander on the R. Colorado

Meander incised into plateau due to rejuvenation

Different strata show evidence of past climates or hydrological events

Different strata show evidence of past climates or hydrological events.

Stratum with large boulders must have formed in wetter conditions when higher river discharge carried a greater competence of load

Migrating meanders, R. Gongola, Nigeria

Former course marked by white sediments

Current channel is braided

Lower Severn Valley

Well developed meanders with bars in the channel indicating high sediment load

Very wide floodplain

Very gentle valley side gradients

River terraces, R. Agri, S. Italy

River has been rejuvenated causing renewed vertical erosion - base level has dropped either due to rising land or falling sealevel

River terraces, R. Agri, S. Italy

Flat terraces represent former floodplains

River has been rejuvenated causing renewed vertical erosion - base level has dropped either due to rising land or falling sealevel

Flooding in the Severn Valley

Floodplain can be mapped into risk zones high to low

Floods can be beneficial - they renew soil fertility by depositing sediment on floodplain

Floods result in large amounts of sediment transported in the channel

Padstow estuary

Estuaries are tidal, dominated by marine sediments

At low tide, mudflats are exposed

Alluvial fan, Buttermere, Lake District

Lake Buttermere acts as a local baselevel for the stream, leading to deposition of load in a triangular shape - an alluvial fan

This is the same process that forms major deltas at the coast e.g. R. Nile

Braided river, Swiss Alps

Daytime snowmelt in summer produces flashy regime

Vast amounts of sediment supplied from frost-shattered valley sides

Braided river, Swiss Alps

Daytime snowmelt in summer produces flashy regime

Vast amounts of sediment supplied from frost-shattered valley sides

Channel course can change daily due to changes in discharge and loose sediments

Summary of channel characteristics

(1) Processes

(1) ProcessesErosion

Corrasion Attrition Solution Hydraulic

action

Headward Vertical Lateral

action

Transport

Suspension Solution Bedload

- traction- saltation

action

Transport

Suspension Solution Bedload

- traction- saltation

Deposition

Summary of channel characteristics(2) Landforms

•Rapids

•Waterfall

•Pothole

•Rapids

•Waterfall

•Pothole

•Meander

•Bluff/cliff

•Point bar

•Pool

•Riffle

•Rapids

•Waterfall

•Pothole

•Meander

•Bluff/cliff

•Point bar

•Pool

•Riffle

•Knick point

•Braids

•Alluvial fan

•Delta

(3) Concepts and technical terms

•Discharge

•Regime - flashy - regular

•Capacity

•Competence

•Discharge

•Capacity

•Competence

•Hydraulic radius

- cross sectional area

- wetted perimeter

•Discharge

•Capacity

•Competence

•Hydraulic radius

- wetted perimeter

•Long profile

•Graded profile

•Rejuvenation

•Base level

•Discharge

•Capacity

•Competence

•Hydraulic radius

- wetted perimeter

•Long profile

•Graded profile

•Rejuvenation

•Base level

•Thalweg

•Helicoidal flow

•Meander migration

Summary of valley characteristics

Upper Slumps Landslides Terracettes (soil

creep) I nterlocking spurs “V” shaped valley

Upper Slumps Landslides Terracettes (soil creep) I nterlocking spurs “V” shaped valley

Middle Wider valley Gentler valley sides Floodplain Meanders

Upper Slumps Landslides Terracettes (soil creep) I nterlocking spurs “V” shaped valley

Middle Wider valley Gentler valley sides Floodplain Meanders

Lower Widest part of valley Terraces

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