Geography - Rivers

• Processes of River Erosin

  
• Processes of River Transport

  
• Factors affecting the work carried out by a river

  • The velocity
  • The volume of the running water
  • The bedrock along the course of the river
• Deposition occurs when a river looses velocity
• Why deposition happens

  • decrease in gradient
  • decrease in the river flow as water drains away after heavy rain
  • the river meeting the sea or lake
  • the river flowing more slowly on the inside of bends
• Discharge


  the volume of water flowing down the river at any one time
• Waterfalls and Gorges

  
• Upper Course features and processes

  • the place where a river starts its course.
  • the river valley at this point is a V-shape in cross-section
• Middle, and Lower Course features and processes

  • the slopes become gentler and the valley widens
  • vertical erosion becomes less important and literal erosion and deposition take over
• The long and cross profile of a river

  
• Cross Profile - shows how the width of the river channel changes across it's path.
• Long Profile - shows how the height of land changes along the length of the river.
• Opportunities presented by rivers

  • flat land which makes it easy to build roads and carry out agriculture
  • soils that are often mineral-rich and fertile
  • river valleys that are often natural route ways
  • the potential to produce hydro-electricity in some cases
• Problems presented by human activity

  • areas suffer form diseases carried by insect pests
  • floodplains and deltas are liable to flooding
• How water reaches a river

  
• Factors affecting discharge: Rainfall, evapotranspiration, river flow, river discharge, river storage, river temperature, river level, river gradient

  
• Managing river flooding

  • Planting Vegetation
  • Reservoirs
  • Straightening the channel
  • Dredging the channel
  • Artificial levees
  • Bridge design
  • Wash lands and building planning
• Managing river floods and impacts

  • PV- trees take in rainwater through roots and lose it by transpiration so it doesn't reach the river
  • RV- water will be trapped on the tributaries and released slowly over a long period of time
  • BD- act like dams which hold back the water and allow it to spread
• How does a waterfall form

  • a horizontal layer of hard rock lies on top of a soft layer of rock
  • the soft rock is eroded more quickly by the river and a plunge pool forms
  • the splashing water in the plunge pool undercuts the hard rock above and eventually the hard rock collapses as there is no support
• Abrasion 


  where sand and rocks are carried along the riverbed
• Hydraulic action


  where fast-flowing water is forced into cracks
• Attrition

  where rocks and stones wear each other away as they knock together, becoming smaller and more rounded
• Solution

  where rocks such as limestone get dissolved in acid rainwater
• Suspension


  where tiny sediments are carried in the river's current
• Solution

  dissolved chemicals are carried along in the river's current
• Saltation

   Small rocks that are too big to be carried out are transported and bounce along the bottom of the riverbed
• Traction

  large rocks are carried along
• Bedload

  heavier material is carried along the bottom
• Meanders and Oxbow Lake formation

  1. Water in a river flows naturally in a corkscrew pattern (helical flow)

  2. Fastest current is forced to the outer bend (A), where it undercuts and erodes the bank to form a river cliff

  3. Helical flow transports sediment from (A) across the channel to the inner bank (B)/ slip-off slope, where the slower-moving water deposits it to a form a point bar

  4. Continued erosion creates a narrow neck btw two meanders (X)

  5. River cuts through the neck at Y, creating a new channel across the meander

  6. Old meander becomes an oxbow lake (Z) when deposition seals the ends - completely separating it from the river
• Potholes

  smoothth, rounded hollows formed in the bedrock of the river bed by vertical erosion
• Rapids

  common features in the upper course of a river. they for at the palces where water is shallow and river bed is rocky and irreguar
• Three types of deposition that help build the floodplain

  • the deposition of point bars on the inside of the meanders, these spread across the valley as the meanders migrate (sideways and downstream)
  • the deposition of gravel on the river bed (part of bed load)
  • the deposition of fine silt and mud on the floodplain itself when the river overflows the banks during floods
• Delta


  an area of low-lying, flat, marshy land where a river meets the sea or a lake
• Causes of river erosion

  • increase in discharge - the greater the velocity of the river the larger rock fragments can be eroded
  • soft rocks - when soft rocks clays and sands of the alluvium are easily eroded
  • channel sizes and shape - erosion may be connected on the outside of the meanders where velocity is greater
• Prevention of river erosion

  • reinforcing river banks - can be done by using concrete, rocks, or wire baskets filled with rocks, known as gabions
  • planting vegetation - reduces run-off and discharge
  • small dams on tributaries - water can be trapped in reservoirs on the tributaries, then released slowly over periods of time