River Regime & Hydrographs (Edexcel IGCSE Geography)

River regimes & storm hydrographs

River regimes

• The discharge of a river is defined as the amount of water passing a specific point on the river at a given time 

• Discharge changes over time

• The river regime is a record of these changes over a year

• Many factors can influence the river regime including:

  • climate

  • vegetation

  • land use

  • geology

  • soil

  • human activity

  • drainage basin size and shape

  • drainage density

  • relief

Storm hydrographs

• A storm hydrograph shows the changes in river discharge after a storm event

• The graph shows a short time period, usually 24 hours

• The storm hydrograph has several features:

  • Base flow which is the 'normal' level of river discharge

    • The water mainly comes from groundwater flow

  • Peak rainfall is the highest rainfall level during the storm

    • The time of the peak rainfall is taken from the centre of the bar

  • The rising limb shows the increase in the river discharge

    • The steeper the limb the faster the river discharge has increased

  • Peak discharge or peak flow is the highest level of discharge

  • Lag time is the time difference between the peak rainfall and the peak discharge

    • The shorter the lag time the higher the risk of flooding because the river may not have the capacity to contain the increase in discharge

    • Where there is more overland flow (surface runoff) the lag time will be shorter

  • The recessional limb shows the river discharge returning to normal flow

    • The steeper the recessional limb is the faster the river returns to normal flow

Factors affecting regimes

• There are many factors which affect the discharge or regime of a river

• The shape of storm hydrographs is also affected by these factors

• Factors which increase overland flow (surface runoff) lead to:

  • shorter lag times

  • increased discharge

  • steeper rising limb

Climate

• Snow and ice melt leads to higher discharge - usually in the spring months

• High temperatures increase evaporation and reduce river discharge

• Higher rainfall often in autumn and winter increases river discharge

• Convectional rainfall in summer or hot, moist climates increases river discharge

Vegetation

• Vegetation increases interception and infiltration leading to reduced overland flow and so lower river discharge

• Deciduous trees lose their leaves in winter decreasing interception, increasing overland flow and river discharge

Land use

• Concrete and tarmac in urban areas and built environments are impermeable leading to high overland flow

  • This is rapidly taken by drainage systems to the rivers/streams, increasing river discharge

Geology

• Permeable rocks increase infiltration and percolation which reduces overland flow and decreases river discharge

• Impermeable rock decreases infiltration and percolation, this increases overland flow and river discharge

Soils

• Soils which are compacted or frozen reduce infiltration, increasing overland flow and river discharge

Abstraction

• Water taken for irrigation and domestic use decreases the river discharge

Dams

• Dams control the flow of water, so can both increase and decrease river discharge

• Reservoirs experience higher levels of evaporation which can decrease river discharge

Relief

• Steep slopes increase overland flow because the water is moving fast, reducing infiltration, this leads to increased river discharge

Drainage density

• High drainage density leads to increased discharge, particularly after a rainfall event