rivers

• The Long Profile


  • River is steepest at its source (start) and becomes gentler as it approaches its mouth (where it meets the sea)
• The Cross-Profile


  • Shows a simple cross-section of the river at certain points of its course
• Features of the Upper Course

  • Waterfalls, gorges, interlocking spurs, large rocks and boulders
• Features of the Middle Course


  • Meanders, floodplains, smaller pebbles and rocks
• Features of the Lower Course

  • Meanders, oxbow lakes, floodplains, levees and fine sand particles
• River and Valley Characteristics - Upper Course

  River flows downhill due to gravity, creating friction and lots of vertical erosion. River channel is narrow and shallow, valley is V-shaped with steep sides.
• River and Valley Characteristics - Middle Course

  Lateral erosion increases as the river becomes less steep. River channel is wide and deep, valley is V-shaped with shallow sides.
• River and Valley Characteristics - Lower Course

  River channel is very wide and deep, valley sides are wide and almost flat, deposition is most evident.
• Why does a river's velocity increase downstream despite its gradient getting shallower?

  The volume of water increases as tributary streams join the river, and the wider, deeper and smoother channel creates less friction, increasing the river's velocity.
• Erosion


  1. Hydraulic action

  2. Abrasion

  3. Solution

  4. Attrition
• Lateral erosion


  Occurs when riverbanks are eroded, making the river channel wider. Most common in the middle and lower courses.
• Vertical erosion


  Occurs when the river bed is eroded, making the channel deeper. Most common in the upper course.
• Transportation

  1. Traction

  2. Saltation

  3. Suspension

  4. Solution
• Deposition


  Occurs when a river loses energy and drops eroded material (sediment). Heaviest rocks and boulders are deposited first, and the lightest materials are deposited last.
• Formation of Interlocking Spurs

  In the upper course, the river winds and bends around hard rock, creating a winding valley of interlocking spurs.
• Formation of Waterfalls and Gorges


  Softer, less dense rock is eroded away faster than the overlying hard rock, causing the hard rock to be undercut and collapse, forming a waterfall that retreats upstream and leaves a steep-sided gorge.
• Rivers: Erosional and Depositional Landforms

  • Meanders
  • Oxbow lakes
• Formation of meanders and oxbow lakes

  1. Erosion on outside bend narrows neck of meander

  2. River breaks through to form new channel

  3. Meander becomes sealed off, forming oxbow lake
• Floodplains


  • Wide, flat areas of land found on either side of a river
  • Extremely prone to flooding
  • Soil is very fertile due to deposition of alluvium
• Levees


  • Naturally raised river banks that form when a river repeatedly floods and deposits sediment
  • Heavier sediment deposited closest to channel, finer sediment carried further over floodplain
• Estuaries


  • Found where tidal mouth of river meets sea
  • Flooded daily by tides
  • Repeated flooding causes layers of sediment to build up, creating mudflats and salt marshes
• Flooding


  Occurs when a river's discharge exceeds its channel's volume, causing the river to overflow
• Factors affecting flood risk

  • Physical causes: Precipitation, Geology, Relief
  • Human causes: Land use
• Hydrograph


  • Shows how a river's discharge changes in response to precipitation
  • Includes: Peak discharge, Peak rainfall, Rising limb, Falling limb, Lag time
• High levels of surface run-off


  Shorten lag time and increase peak discharge, resulting in high flood risk
• High infiltration rates


  Increase lag time and reduce peak discharge, lowering flood risk
• Soft engineering


  Managing natural river processes to reduce the flooding risk
• Soft engineering strategies


  • Flood warnings and preparation
  • Flood plain zoning
  • Planting trees
  • River restoration
• Flood warnings and preparation

  1. Flood alert information issued by agencies

  2. Warnings give people time to evacuate and protect their homes

  3. Warnings are only effective if people are aware and take action
• Flood plain zoning


  • Restricts building on flood plains and areas at risk from flooding
  • Reduces risk of flooding in flood plains
  • Restricts industrial and urban development, which can increase housing shortages
  • Has no effect on existing high-risk urbanised areas
• Planting trees


  • Increases interception and absorption of rainwater, reducing surface run-off
  • Low-cost, environmentally friendly way to reduce surface run-off
  • Increases natural wildlife habitats and reduces soil erosion
  • Requires a lot of space and reduces land available for farming
• River restoration


  • Removes hard engineering strategies to allow rivers to return to natural state
  • Increases water storage, reducing downstream flooding risk
  • Makes river more aesthetically pleasing and increases biodiversity
  • Can lead to loss of agricultural land and cause major flooding if done near high-value, built-up areas
• Hard engineering


  • Use of man-made structures to control river flow and prevent flooding
  • Expensive but effective flood management option
• Hard engineering strategies


  • Dams and reservoirs
  • River straightening
  • Embankments
  • Flood relief channels
• Dams and reservoirs


  • Trap water, forming artificial lake or reservoir
  • Can be used as source of drinking water and to generate hydroelectric power
  • Very expensive and can flood large areas of land, damaging habitats and displacing people
  • Can prevent transportation of eroded material, reducing farmland fertility downstream
• River straightening


  • Cuts out meanders to create straighter, wider and deeper river channels
  • Reduces friction and river length, allowing water to flow out of area more quickly
  • Increases downstream flooding risk as water is carried there more quickly and causes greater erosion
• Embankments


  • Artificially raised banks built alongside rivers
  • Enable rivers to hold more water, protecting surrounding towns and cities
  • Expensive and can be unsightly
  • Devastating floods can result if embankment fails or water level rises above banking
• Flood relief channels


  • Divert water away from urban areas or redirect water if river level becomes too high
  • Reduce flooding risk in urban areas
  • Costly, and could overflow and cause severe flooding if river discharge increases significantly