Rivers

• What is a Meander?

  A meander is a wide bend of a river found mainly in lowland areas. They constantly change their shape and position due to lateral erosion and deposition. The water flows at different speeds, slower areas are found in the deeper parts of the river, shallower areas have slower moving water.
• Thalweg:


  The line of fastest flow in a river
• Pool:


  deep, fast moving sections of the river
• Riffles:


  shallow, slow moving sections of the river
• Slip-off slope:


  the inside bank of a meander
• River cliff:


  the outside bank of a meander
• The first step in the development of an oxbow lake is:

  • The river starts to flow from side to side in a winding course but still in a relatively straight channel, the pools tend to move to opposite sides of the channel over time and this is where a meander will develop.
• The second step in the development of an oxbow lake is:

  • Water moving faster has more energy to erode, this occurs on the outside of the pool
• The third step in the development of an oxbow lake is:

  • water moving slowly tends to deposit material as it has little energy left to erode or transport material, this occurs on the inside of the bend.
• The fourth step in the development of an oxbow lake is:

  • meanders migrate downstream and change shape over time
• The fifth step in the development of an oxbow lake is:

  • the neck of land between the loops gets narrower
• The sixth step in the development of an oxbow lake is:

  • at the next flood, the river may cut through this to find a faster course, this then deposits material at the entrance to the old meander, cutting off the flow of water and creating an oxbow lake.
• Discharge:


  the quantity of water that passes a given point on a stream of river-bank within a given period of time
• Levees: 


  Embankments of sediment along the banks of a river
• Flood plains:


  the relatively flat area forming the valley floor on either side of a river channel, which is sometimes flooded.
• River bluff:


  a very steep and broad hill or small cliff next to a river
• Levees are raised river banks (2-8m in the UK) composed of gravel, stones and alluvium (loose soil or sediment). They are steep sided but steeper on the channel side than the land side. they are formed by flooding over many years.
• Floodplains are wide flat areas of marshy land on either side of the river, meanders migrate across floodplains due to lateral erosion. When they reach the edge of the floodplain they rode the valley side (bluff). When the river floods it deposits silt, creating a very flat floodplain, layer upon layer builds up over many years to form a thick deposit of fertile alluvium.
• Estuaries are transitional tidal zones between the river and coastal environments, they are affected by wave action as well as river processes. The main process operating is deposition, during a rising tide, the river water is unable to be discharged into the sea. The river velocity falls and sediment is deposited, this deposition forms mudflats which are visible at low tide.
• What is flooding?


  Flooding is where land that is not normally underwater becomes inundated. A river flood occurs when a river channel can no longer hold the amount of water flowing into it, water overtops the banks and floods the adjacent land.
• Causes of flooding:

  • river flood usually occur after a long period of rainfall, often during winter
  • sudden floods can occur during torrential storms, these are called flash floods, they are often associated with heavy rainfall that happens in the summer
• Physical factors that affect flooding:

  • Precipitation- torrential rain can lead to flash floods and prolonged heavy rainfall can lead to flooding as river capacity is exceeded
  • Geology (rock type)- impermeable rocks encourage overland flow speeding up water flowing into rivers, increasing the flood risk
  • Steep slopes- these encourage rapid transfer of water into rivers, therefore increasing the flood risk
• Human factors that affect flooding:

  • Lack of river management- failing to dredge rivers regularly leads to decreased river capacity
  • Urbanisation- building on a floodplain creates an impermeable surface so water transfers quickly through drains into rivers
  • Agriculture on flood plains- if soil is left unused and exposed, surface run off increases, soil transfers into river, reducing the channel's capacity
  • Deforestation- trees intercept water, if they are removed water flows more quickly into rivers
• Hydraulic action:

  Force of the water hitting the river beds and banks. Most efficient when a lot of water is moving fast.
• Abrasion:

  load carried by the river bashes into the bed and banks dislodging particles into the flow of the river.
• Attrition:

  When stones carried by the river knock into each other, gradually making them rounder and smaller.
• Solution/ corrosion:

  When the river flows over limestone o chalk, the rock is slowly dissolved (they are soluble in slightly acidic water)
• Erosion:

  The wearing away and removal of rock
• Weathering:

  The wearing away and break down of rock in situ
• Traction:

  Large boulders and rocks are rolled along the river bed
• Bedload:

  Load carried on the river bed, usually larger and heavier rocks
• Saltation:

  Small boulders and stones are bounced along the river bed
• Suspension:

  fine, lightweight material (such as alluvium) is held uo and carried by the river's flow
• Solution:

  Minerals are dissolved in water, this affects rocks and as such its a chemical change