Sec 2 rivers 2013

Rivers

SEC ONE TEXTBOOK PAGE 101

What is a river?

• A river is a natural watercourse, usually freshwater, flowing towards the sea.

• Without the hydrological cycle, rivers cannot be formed.

HYDROLOGICal CYCLE

HYDROLOGICAL

CYCLE

SURFACE RUNOFF

INFILTRATION

CONDENSATION

TRANSPIRATION AND EVAPORATION

PRECIPITATION

HYDROLOGICAL CYCLE

• Precipitation – water from clouds come to the Earth’s surface as rain, snow and hail• Transpiration – plants release water vapour into atmosphere

HYDROLOGICAL CYCLE

•Surface Run-off – rain that falls on Earth’s surface flows over land

• Condensation – water vapour rises, cools and changes into tiny water droplets

HYDROLOGICal CYCLE

• Infiltration – water that soaks into the soil subsurface

• Percolation – water moves further down into the underlying soil after infiltration

HYDROLOGICal CYCLE

•Throughflow – infiltrated water that flows sideways in the soil subsurface

• Interception – water is absorbed by trees

RIVER SYSTEM

• Source – the starting point of a river• Mouth – the ending point of a river• Drainage Basin – The area drained by a river and its tributaries

RIVER SYSTEM

•Confluence – the point at which a tributary joins the main river • Watershed – The boundary formed by the edges of the drainage basin

Waterfalls

• Sudden, steep and vertical flow of fast-flowing water falling from great heights

• Formed by river erosion

Niagara Falls on Niagara River, Canada and USA

• formed by erosion of rock of different resistance

Formation of Waterfalls

Formation of Waterfalls• River flows across rocks of different

resistance.

Formation of Waterfalls

• River erodes the less resistant rock more rapidly• This cause a change in the gradient of the river

course.

Formation of waterfalls• Over time, river plunges from a great height to

hit the riverbed below, • This forms a waterfall.

Formation of plunge pools

• Repeated pounding of the river bed leads to a depression at the base of a waterfall

• A plunge pool is formed.

Meander

• A river that follows a winding course, usually forming hoop-like bends.

• River may meander because

Lower gradient lower speed lower river energy river turns to avoid obstacles

Meander

Note: Outer Bank = Concave Bank

Note: Inner Bank = Concave Bank

River Cliff

• Constant erosion (undercutting) erodes the outer (concave) bank of the meander

• forming a river cliff.

Slip-off slope

River cliff

Slip-off slope

• Constant deposition at the outer (convex) bank deposits sediments

• forming a slip-off slope.

Slip-off slope

River cliff

Floodplain

Levees

FloodplainAreas of low-lying ground adjacent to a

river at the lower course

Formation of Floodplain and Levee

Formation of Floodplain and Levee

Floodplain and Levee• During a heavy rainfall, river water

overflows its bank. River loses speed as it encounters friction.

Floodplain and Levee• Deposition occurs as river’s energy decreases. • Coarser and heavier sediments are deposited

nearer to the river bank. • Finer and lighter sediments are deposited

further away.

Formation of Floodplain and Levee

Floodplain and Levee• Overtime, the accumulation of fine

sediments results in the formation of flat plains

• Coarser and heavier sediments are acculumated

• Results in the formation of raised banks • Known as levees.

Formation of Delta• At the river mouth, a river loses energy and starts to deposit its sediments.

Formation of Delta• As the

sediments build up, the mouth will be blocked

• The river also split into several small channels called distributaries to flow to the sea.

DistributariesDelta

Over time, the accumulated sediments that rise over the water and the distributaries form a delta.

Positive Impact of river

• Rivers provide water for domestic purposes such as drinking, bathing and washing.

• Rivers are a good source of food such as fish and prawns.

Positive Impact of river

• Rivers are important waterways for transporting people and goods.

Negative Impact of river

• River water overflows river bank cause lives to be lost and destroy property and crops.

• .

Negative Impact of river

• Consuming contaminated river water can cause people to fall ill

• Eg contract diseases such as cholera.