River Landscapes

Drainage basin

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Cross Profiles

Upper course-

  • source, steep gradient
  • valley- steep sided, V shaped
  • river- narrow, shallow turbulent

Middle course-

  • gentle gradient
  • valley- wider, flat floor
  • river- wider and deeper

Lower course-

  • very gentle gradient, mouth
  • valley- very wide and flat
  • river- wide, deep, with large sediment load
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DB Key Words

Drainage basin- the area of land drained by a river and its tributaries

Source- the start of the river

Tributary- a small stream that joins a larger river

Mouth- the end of a river, usually where it joins the sea

Watershedthe edge of a river basin

Long profile- the gradient of the river, from its source to its mouth

Cross profile- the side-by-side cross section of a river channel and/or valley (downstream)

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How does the cross profile change?

Changes are due to the amount of water flowing in the river

As tributaries add more water (and energy) to the river, it erodes its channel, making it wider and deeper

Changes are mainly due to channel erosion, broadening and flattening the base of the valley

Together with weathering and mass movement, these processes mske the sides of the valley less steep

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Types of Erosion

Vertical- downwards

Lateral- sideways

Hydraulic action: the force of the water hitting the river bed and banks. Most effective when water is moving fast and at a high volume

Abrasion: the load carried by the river hits the bed or banks, dislodging particles

Attrition: stones carried by the river knock against each other, becoming smaller/ more rounded

Solution: alkaline rocks e.g. limestone, are dissolved by slightly acidic river water

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Deposition

When a river's velocity DECREASES, and it no longer has the energy to transport its load, it DEPOSITS it

  • Larger rocks: transported mainly by TRACTION are only carried SHORT distances during periods of HIGH FLOW, deposited in UPPER COURSE
  • Smaller sediment: carried further DOWNSTREAM mostly in SUSEPNSION, deposited on a river's BED and BANKS where velocity SLOWS due to friction

Lots of depostion occurs at MOUTH where velocity REDUCES because of GENTLE GRADIENT and INTERACTION with tides

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Transportation

Load- material transported by the river

The size and the amount of load carried depends on a river's speed or velocity

e.g. after heavy rain, the river looks muddy and carries lots of sediment quickly

Solution- dissolved load

Suspension- small sediment held in the river

Tractionlarge particles rolled on the river bed

Saltation- 'bouncing' of particles too heavy to be suspended

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Course deposition

Upper: erosion landforms e.g. waterfalls

Middle: mostly erosion and deposition ladnforms e.g. meanders and transportation

Lower: mostly deposition landforms e.g. levee

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River Landforms- Waterfalls

Waterfalls- as a river flows downstream, it crosses different rock types. More resistant rocks are less easily eroded than less resistant rocks, froming steps in the river'a long profile, which form waterfalls.

Also form when:

  • sea levels drop causing river to cut down into its bed creating a step (called a knick point)
  • in glacial hanging valleys- a tributary glacial trough on the side of a main valley often with a waterfall)
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Gorges Diagram

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Waterfall Diagram

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River Landforms- Gorges

Gorge- a narrow, steep-sided valley found downstream of a retreating waterfall.

They can form in other ways:

  • at the end of the last glacial period, masses of water from melting glaciers poured off upland areas forming gorges (e.g. Cheddar Gorge, Somerset)
  • on limestone, when large underground caverns can accomodate an entire river
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Meanders Diagram

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River Landforms- Interlocking spurs

Interlocking spurs- mountain stream erodes vertically creating a V-shaped valley. It winds around areas of resistant rock to create them Image result for interlocking spur

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River Landforms- Meanders

Meanders- bends in a river found mainly in lowland areas. They constantly change shape and position. 

The thawleg is the line of the fastest current- it swings from side to side causing erosion on the outside bend, and deposition on the inside bend. These processes cause meanders to migrate across the valley floor.

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River Landforms- Ox-bow Lakes

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Ox-bow lakes- As meanders migrate across the valley floor they erode towards eachother

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River Landforms- Floodplains

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Floodplains- wide, flat areas on either side of a river in its middle and lower courses. They are created by migrating meanders and floods depositing layers of silt to form alluvium.

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River Landscapes- Levees

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Levees- when, in low flow, deposition raises the river bed so the channel can't carry as much water. During flooding, water flows over the sides of the channel. As velocity decreases, coarser sediment is deposited first on the banks- then finer sand and mud, raising the height of the levees.

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River Landforms- Estuaries

Estuary- where the river meets the sea. They are affected by tides, wave action and river processes. As the tide rises, rivers can't flow into the sea, so velocity falls and sediment is deposited forming mudflats, which develop into salt marshes.

Image result for estuaries

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