Geography - Rivers

Geography - Rivers

1. How easily can a drainage basin be impacted?

   • Human impacts on the hydrological cycle
     • CO2 emissions
       • It interrupts the rainfall patterns, this leads to more evaporation
     • Dams
       • Stops the flow of water so it builds up and then floods
     • Chemicals
       • Polluting the water which can cause animals and humans to die
     • Ubranisation
       • Building roads and buildings means less infiltration and more surface run-off causing flooding
     • Overabstraction of groundwater
       • This can lead to desertifacation as there isn't any water
     • Desforestation
       • There is less interception which causes more surface run-off and leading to flooding
   • Stormhydrographs
     • Lag time
       • This is the delay between peak rainfall and peak discharge
     • Rising limb
       • This is the part of the graph up to peak discharge
     • Peak discharge
       • This is the highest point on the graph, when the river discharge is at its greatest
     • Falling limb
       • This is part of the graph after peak discharge
   • Key words
     • Percolation
       • It is water seeping down through the soil into the water table
     • Evapotranspiration
       • It is the process of evaporation and transpiration
   • Water balance
     • Water balance is worked out from inputs (precipitation) and output (channel discharge and evapotranspiration)
       • The water balance affects how much water is stored in the basin
     • General water balance in the UK
       • In wet seasons, precipitation exceeds evapotranspiration. This creates a water surplus
         • The ground stores fill with water so there's more surface runoff and higher discharge, so river levels rise
       • In drier seasons, precipitation is lower than evapotranspiration. Ground stores are depleted as some water is used and some flows into the river channel, but isn't replaced by precipitation
       • So, at the end of the dry season, there's a deflicit (shortage) of water in the ground. The ground stores are recharged in the next wet season (i.e. autumn)

2. How do rivers and valleys change downstream?

   • Erosion
     • Hydraulic action
       • The pressure of the water breaks the particles from the river bedand banks. Water pushes into the cracks, air becomes compressed and the bank collapses
     • Corrasion
       • The sediment carried by a river scours the bed and banks, this causes pebbles to spin around and turn into potholes
     • Attrition
       • Eroded rocks collide and break into smaller fragements, the edges become smooth and rounded
     • Corrosion
       • CO2 dissolves in the river to form a weak acid, this dissolves the rock
   • Transportation
     • Suspension
       • The materials are made up of very fine particles, that are lifted as a result of turbulance and transported by the river
     • Solution
       • When material is dissolved and carried by the river
     • Saltation
       • When material is too heavy to be carried in suspension it is bounced along the river by the force of the river
     • Traction
       • When large materials are rolled and pushed along the river bed by the force of the river
   • What influences velocity
     • Channel shape
     • Roughness of the channel bed
     • Banks of the river
     • Channel slope/gradient
   • Hydraulic radius
     • The ratio between the cross sectional area and the length of the wetted perimter
     • This measures the efficiency of the river at moving water
   • Compentence
     • Is the maxium size of load a river is capable of transporting
   • Capacity
     • Refers to the total volume of sediment a river can transport
   • What factors affect capacity and competence?
     • Velocity
     • Long profile
     • Thalweg
       • Lowest line of elevation in the river valley
   • Hjulstrom Curve
     • This graph represents the realtionship between erosion, transportation and deposition of sediment.

3. Do all rivers look the same?

   • Deltas
     • Birds foot
       • Spread over a wide area with many 'distributaries' extending out to sea
         • E.G Mississippi
     • Factors that affect the formation of deltas
       • How rough the sea is
       • Volume of water
       • Changes in the coast level
       • How much sand has been eroded away
     • Positives of humans living near deltas
       • Fertile land
       • Good fishing grounds
     • Negatives of humans living near deltas
       • Unstable grounds
   • Estuary
     • It is where fresh water and salt water meet/interact
   • Rejuvenation
     • Over long periods of time large changes in sea level can drastically alter the rivers potential energy and can increase the chances of vertical erosion
     • Keywords
       • Incised meanders
         • If a rejuvenated river occupies a valley with well-developed meanders, renewed energy results in them becoming incised or deepend
       • Knickpoints
         • A location of a river, where there is a sharp change in channel slope, such as a waterfall or lake
       • River terraces
         • A rejuvenating river can erode vertically into the former flood plain to produce river terraces

4. Erosional fluvial features

   • Floodplains
     • An area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding
     • They are meant to flood but so many people build their houses on them because there isn't any room
   • Levees
     • It is an embankment built to prevent overflow of a river
     • Levees help communities to reduce the risk of flooding
   • Braiding
     • A braided river is a river that had multiple channels and islands of sediment in between those channels
   • Deltas
     • Deltas are depositional landforms, they are created by loading of sediment onto the land as the river can't carry it anymore
   • Waterfalls
     • Waterfalls often form in the upper stages of a river where it flows over different bands of rock. It erodes soft rock more quickly than hard rock and this may lead to the creation of a waterfall. Formation of a waterfall: The soft rock erodes more quickly, undercutting the hard rock

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