rivers

• The Drainage Basin System


  - part of a hydrological (water) cycle that operates on land

  - made up of inputs, flows, stores and outputs

  - input - water entering the system

  - transfers (flows) - the movement of water through the system

  - stores - water stored in the system

  - outputs - water leaving the system

  
• dbs definitions

  surface storage - when water lies on the ground as puddles or lakes

  precipitation - rain, snow, sleet, or hail that falls to the ground.

  groundwater flow - very slow water movement deep below the ground

  evaporation - when water heated by the sun changes to water vapour

  infiltration - when surface water soaks into the soil

  interception - when water droplets collect on trees and plants

  through-flow - water flowing downhill through the soil

  surface runoff - water moving across the ground as a steam and river

  soil moisture - water stored in the soil and broken rocks near the surface

  percolation - water moving downwards through the soil into rocks below

  groundwater - water stored in porous rocks deep below the ground

  transpiration - water vapour by tress and plats

  water table - upper level of saturated rock/soil
• what is a dbs?


  a drainage basin is the area of land drained by a river and its tributaries

  watershed - the boundary diving one drainage basin from another - a ridge of highland

  mouth - where the river flows into the sea or sometimes a lake

  confluence - the point at which two rivers join

  tributary - a river which joins a larger river

  source - the upland area where the river begins

  catchment - the area from which water drains into a particular drainage basin

  
• river erosion process


  river erosion is the wearing away of the land as the water flows past the bed and banks, as well as eroding its own channel the river may also erode into the surrounding valley sides which may lead to mass movement

  abrasion - this is caused by the scraping away of the river bed and banks by stones picked up and carried in the rivers flow

  hydraulic action - this is where the force of the water in the channel hitting against the bed and banks gradually wears away - particularly occurs when high - velocity flow

  corrosion - the dissolving of minerals in the rocks in the bed and banks which are carried away in solution

  attrition - this occurs as rocks being against each other, gradually breaking each other down
• river transport processes


  the material carried by a river is called load

  this load is transported by water in one of a few ways

  the size of material and the velocity of the water controls the way a river transports material

  
• deposition


  - the process where a river is transporting material and can no longer be carried and is dropped. material dropped by a river is called sediment

  - the two main factors affecting deposition are the amount of energy and the size of the material

  - a river will deposit material when it loses energy

  - in large rivers such as the Nile and the Mississippi rivers may form deltas where they meet the sea due to large amounts of deposition

  - estuary - the tidal mouth of the river
• where could deposition occur


  - where a river enters a shallow area

  - where a river meets the sea

  - after a period of drought when there is less water in the river

  - on the inside bend of the meander where water flows more slowly
• courses of rivers definitions


  - gradient - the change in slope of the river bed as it passes from source to mouth

  - channel width - the distance from one beak to another

  - channel depth - the distance from the surface of the water to channel bed

  - velocity - the speed at which water is moving through the channel measured in m/s

  - discharge - the amount of water passing a given point in a given time - measured in cumecs (meter^3/second) calculated by velocity x volume
• a rivers long profile


  - shows how the river gradient changes as it flows from source to mouth

  - it starts steep and reduces with distance from the source. it has a concave profile

  
• how and why do the channel width and depth change downstream?


  - downstreams it increases and the reason for this is erosion will occur which deepens and widens the channel

  - in the upper course the river cuts into its bed as it moves down to the middle course it erodes into its banks
• how and why does the gradient change downstream?


  - decreases downstream because as the river flows downstream as it passes from the upland area down to the lowland area where it enters the sea
• how and why does the velocity change downstream?


  - increases downstream as the river gets deeper the speed flow can increase as there is lelss friction as there is less contact between the river and the banks
• how and why does the discharge change downstream?

  - increases downstream as the river goes downstream more dributaries feed into the velocity and channel volume is also increase so discharge increases
• how and why do vally cross profiles change downstream?


  - a river valley is a low area or depression which has a river running through it

  - as a river runs from its source to its mouth, the shape of the valley changes significantly with regards to the steepness of the valley sides and the area round the river itself

  
• changes in a valley cross profile with distance downstream - upper course


  - description - a steep-sided, narrow and deep v-shaped valley with little space around the river

  - explanation - vertical erosion is dominant creating a slope, weathered material moves downslope creating a v-shape and harder rock means the slope stays steep

  
• changes in a valley cross profile with distance downstream - middle course


  - description - valley becomes wider with a small flood plain and more gently valley sides

  - explanation - river flows through a lower area with a less steep gradient, the river begins to meander as lateral erosion occurs - widens the valley and more weathering and erosion as the rock is softer

  
• changes in a valley cross profile with distance downstream - lower course


  - description - large expanse of the flat flood plain either side of the river - very wide and flat valley

  - explanation - gradient is low as river is reachinbg sea level, deposition is building up the flood plain and meanders migrate across the flood plain widening the valley

  
• channel cross profile


  - this relates to the cross-sectional shape of the channel or valley - including width/depth and shape

  
• why does the channel cross-profile change downstream?

  - as the river goes downstream more tributaries join it. these give more energy, a hugher velocity and more erosive power. this results in channel becoming deeper

  - the river channel becomes wider downstream as the gradient is less therefore there is less vertical erosion and more lateral erosion
• how has river erosions shaped the landscape?


  - rapid vertical erosion in the upper course forms a number of distinctive upper course landforms:

  - v-shaped valleys and interlocking spurs

  - waterfalls

  - gorges
• waterfalls and gorges


  - a waterfall is a vertical drop in the channel, representing a change in the slope of the river

  - hydraulic action - the sheer force of water attacking cracks in the rock

  - corrosion - rocks in the water rubbing against the bed and banks

  
• interlocking spurs


  - these are projections of high land that alternate from either side of a valley and project into the valley floor

  - characteristics are steep gradient, convex slopes, a project from the valley side, may have scree slopes, form a zip-like pattern
• characteristics of v shaped valleys


  narrow

  steep sided

  have interlocking spurs
• how are interlocking spurs formed?


  1. the river uses its load to cut down into the bed rock (causing vertical erosion)

  2. material loosened by weathering on the valley sides washed into the river increasing the load and the ability to erode

  3. the river gradually takes a winding path due to obstacles of hard rock in its path - this forms the interlocking spurs as the rivers is forced to wind around these hard rock projections
• how are waterfalls formed?


  1. they usually occur where a river crosses a band of less resistant rock flowing over resistant rock creating a small step over which water will flow as a vertical drop

  2. as the river passes over the hard rock, the soft rock is eroded more quickly than the hard roc and gradually the hard rock, is undercut leaving it elevated above the stream bed

  3. the drop gets steeper as the river continues to erode the soft rock beneath the abrasion and hydraulic action. the force of the falling water creates a plunge pool at the base

  4. further undercutting of the hard rock and enlargement of the plunge pool occurs due to abrasion and hydraulic action. eventualy the cap rock becomes unsupported and collapses

  5. fallen rocks continue to enlarge the plunge pool by abrasion

  6. a steep-sided valley known as a gorge is left behind as the waterfall retreats upstream and the process continues again.
• characteristics of a gorge


  - a very narrow valley

  - steep, high valley sides

  - downstream of a waterfall

  - fast-flowing water

  - bare rock sides

  - boulders in the river bed (from collapsed cap rock)

  - as a waterfall retreats upstream it leaves a steep - sided valley. everytime the overhanging cap-rock collapses, the gorge grows longer
• meander


  meanders are wide bends of a river found mainly in lowland areas. meanders change their shape and position. meanders move sideways in a lateral movement causing erosion and deposition of the river channel.

  
• meander in lower course


  - a meander is asymmetrical in cross-section. it is deeper on the outer bend and shallower on the inside bend

  - in the lower course of the river meander bends become even more extreme and gradually the neck of the meander narrows eventually forming distinctive features known as ox-bow lakes
• ox bow lake formation


  - the neck of the meander gradually erodes with deposition on the inside and erosion on the inside

  - the neck of the meander gets narrower and depositions seals off the old meander and now the water takes the quickest route

  - and oxbow lake is left behind when the meander is cut off

  
• characteristics of floodplains


  - large area of flat land either side of a river

  - wide flat area of land
• characteristics of leeves


  - raised river banks (about 2-8m high in the UK)

  - composed of gravel stones and aluminium

  - grading of sediments with coarsest closest to the river channel

  - steep sided(on channel side)

  - fairly flat top(naturally covered by grass)
• explain floodplains


  - floodplains are made up of layer of sediment deposited over time by a series of floods due to increase friction during flood, flow slows, and deposits materials

  - as meanders cut across the floodplain they widen it

  - after each flood, the floodplain is left a little higher up due to the build up of sediment
• how are leeves formed?


  before flood - water held within the channel

  thickest and coarsest sediments deposited at channel edges

  thin and fine sediments deposited over outer parts of floodplain

  after flood - as water passes over surface increased friction and flow slows - heaviest material deposited first and closest to the river (more difficult to carry). finer sediments travel further builing up the floodplain

  leeves get higher after succesive floods
• physical factors that can increase flood risk


  - precipitation - severe rainstorms can lead to sudden flash floods as river channels cannot contain the sheer volumer of water flowing into them. steady rainfall over several days can also lead to flooding in lowland river basins

  - geology - impermeable rocks (rocks that do not allow water to pass through them) such as sales and clays encourage water flow overload and into river channels. this speeds up water flow and makes flooding more likely

  - steep slopes - in mountain environments steep slopes encourage a rapid transfer of water towards river channels. this increases the risk of flooding
• human factors that can increase flood risk


  - urbanisation - building on floodplain creates impermeable surfaces such as tarmac roads, concrete driveways and alte roofs. water is transferred quickly to drains and then into urban river channels. this rapid movement of water makes flooding more likely

  - deforestation - much water that falls on trees is evaporated or stored temporarily on leaves and branches. trees also use up water as they grow. when trees are removed much more water is suddenly available and transferred rapidly to river channels, increasing flood risk

  - agriculture - in arable farming, soil is left unused and exposed to the the elements for periods of time.this can lead to more surface runoff. this is increased if the land is ploughed up and down steep slopes, as water can run quickly along the furrows
• hydrographs


  - a graph which shows how river discharge changes in response to precipitation event

  - rising limb - rising flood water in the river

  - peak flow - max discharge in river

  - recession limb - falling flood water in the river

  - basin lag time - time difference between the peak of the rain storm and the peak flow of the river

  - base flow - normal discharge of the river

  
• hard engineering


  - man made structures used to prevent or control natural processes from taking place
• soft engineering


  - adapting to a river and working with natural processes