River

Hydrological characteristics

• Drainage basin – area of land where water flows into river
• Watershed – line which divides one river basin from another
• Source – start of river
• Tributary – small river which flows into main river
• Confluence – where tributary joins main river
• Mouth – end of river where it meets sea/lake

Water cycle

• Evaporation, Condensation, Precipitation
• Interception – precipitation caught by the vegetation
• Transpiration – water from vegetation transferred back to atmosphere
• Overland flow (surface runoff) – water flowing over ground surface
• Throughflow – sideways movement of water through soil towards river
• Groundwater flow – sideways movement of water through permeable rock towards river
• Infiltration – downward movement of water into soil
• Percolation – downward movement of water through permeable rock

River characteristics

• Long profile – gradient change source → mouth
• Cross profile – shape across valley
• Cross section – shape across river channel
• River discharge – amount of water flowing, width × death × velocity
• Load – material which river is carrying

– boulders → cobbles → pebbles → gravels → sand → slit → clay (sequential deposition)

Course	Valley	River	Landforms
Upper	Narrow V-shaped Steep sided	Narrow & shallow Steep gradient Load large & angular Low velocity & discharge Vertical erosion – downward erosion of river bed	V-shaped valleys Interlocking spurs Waterfalls Potholes Rapids
Middle		Lateral erosion – sideway erosion of river bank	Small meanders Flood plains
Lower	Wide Less V-shaped Gentle sided	Wide & deep Gentle gradient Load small & rounded High velocity & discharge Lateral erosion	Meanders Flood plains & levees Deltas Oxbow lakes

River process

Erosion

• Hydraulic action – force of water compress air in cracks in rocks & wear away loose materials
• Abrasion – load carried by river collide & wear away bed & banks
• Solution – acid in water reacts & dissolves carbonation rocks & carried away in solution eg limestone, chalk
• Attrition – load carried by river collide with each other, become smaller & smoother

Transportation

• Traction – large materials rolled along bed
• Saltation – lighter materials being picked up by water & bounced along bed
• Suspension – smallest & lightest materials carried along water
• Solution – dissolved materials carried along water in solution

Why deposition occurs? Low velocity, not enough energy to carry loads

Landforms

V-shaped valleys

Formation

1. River erode vertically into its bed by hydraulic action & abrasion
2. Deepens & widens valley, steepens valley sides

Waterfalls

Features  – Overhang hard rock, vertical drop, white water, rocky, plunge pool

Formation

1. Horizontal layer of hard rock (eg granite) on top of layer of soft rock (eg sandstone)
2. Water falls down → form plunge pool below waterfall
3. Soft rock is eroded quicker → undercutting beneath hard rock
4. Unsupported hard rock collapses into plunge pool
5. Process repeats itself
6. Waterfall retreats upstream → form gorge & leaves steep-sided valley

Potholes

Formation

1. Stone trapped in water currents
2. Erode small holes in bed
3. Stones trapped in holes
4. Water swirl them around in hole
5. Erode & make holes larger & deeper

Meanders

• Move downstream coz high velocity erode outer bend

Formation

1. Water turns around obstructions eg stones → faster & slower water movement
2. Outer bend – high velocity & energy for lateral erosion → forms river cliff
3. Inner bend – low velocity & energy → deposition, forms point bar on slip off slope

Flood plain & levees

Formation

1. River erodes valley sides → form wide, flat valley
2. When flood, water shallower → ↑ friction ↓ velocity
3. River loses energy & deposits load → form flood plain & alluvium
4. Heavier load deposit first on banks → form levees
5. Lighter load deposit further away → build up levees
6. Process repeats → build up flood plain & levees

Deltas

Features

• Distributaries
• Formed by 2+rivers
• Shape – Arcuate / bird’s foot

Formation

1. When river enters sea → deposition
2. Salt water causes flocculation (when river meets sea, salt cause clay particles stick together & become heavier → deposit)
3. Deposited loads builds up in layers → form deltas
4. River splits into many small channels (distributaries)

Oxbow lakes

Formation

1. Faster flow on outer bends of meander
2. Erosion on outer bends
3. Neck of meander narrows
4. Eventually due to erosion / flood, river cut through neck, water flows straight across
5. Deposition seal former meander → form oxbow lake
6. Overtime, stagnant water in oxbow lake dry up → form meander scar

Cause	Prevention
Soil & rock saturated / Water not fast enough to infiltrate Too much water runs off to river Channel fills & overflows	Afforestation – ↑ interception Dredging – deepen channel → hold more water Build artificial levees – (same) Dams – store water, control river discharge

Case Study – River Ganges & deltas in Bangladesh

Major floods happen from June to Sept

 

Physical causes	Human causes
Many large rivers eg Ganges flow through Bangladesh with high discharge Low lying land Snowmelt from Himalayas & heavy monsoon rains in Summer	Deforestation Urbanization Poor flood defense – artificial levees collapse when high discharge Global warming – Snowmelt from Himalayas
Impact (in 2007)	Reduce impact
Over 1100 ppl died House damaged – 10 million ppl displaced/homeless 2/3 of land flooded 2 million acres of agricultural land damaged $300 million crop damaged Contaminated drinking water – diseases eg cholera	Flood Action Plan set up in 1989 Built 350km of artificial levees Built 7 dams Negotiation between gov of Bangladesh & Nepal → ↓ deforestation ↑ afforestation Dredging Warnings, evacuation plan

Opportunities

• Flat land – easier to build settlements
• Agriculture – water used to irrigate crops

                     – deposited alluvium → fertile soil

                     – high crop yield, stable food supply

• Water supply – agricultural, domestic & industrial use
• Trade – river used for transportation of goods → encourage trading
• Fish / shrimp – source of protein