The amount of water in a river fluctuates due to a number of reasons

Hydrological cycle

Precipitation: any source of moisture reaching the ground (e.g. rain, snow, hail)

Interception: water being prevented from reaching the surface by trees or grass

Surface storage: water held on the ground surface (e.g. puddles)

Infiltration: water sinking into soil/rock from the ground surface

Soil storage/soil moisture: water held in the soil layer

Percolation: water seeping deeper into rock

Groundwater: water stored in rock

Transpiration: water lost through pores in vegetation

Evaporation: water lost from ground

Surface run-off: water flowing on top of the ground

Throughflow: water flowing through the soil layer

Groundwater flow: water flowing through the rock layer

Water table: current upper level of saturated rock/soil where no more water can be absorbed

Drainage basin

Source – The beginning of the River
Confluence – Where two rivers meet
Watershed – An area of high land between two drainage basins
Mouth – Where a river flows into a lake or the sea
Tributary – A small river or stream that flows into a larger river
Drainage Basin – The area drained by a river and its tributaries

Storm Hydrographs

A storm hydrogaph is used to show how a river responds to a period of rainfall. The shape of the graph helps to you decide whether that basin is likely to flood or not. A graph will a short lag time and a high peak discharge is more likely to flood than one with a longer lag time and lower peak discharge.

Discharge – this is the amount of water in a river at any given point and time. Discharge is measured in cumecs (cubic metres per second)
Velocity – speed of a river (measured in metres per second)
Hydrograph – a graph showing changes in river discharge over time in response to a rainfall event.
Lag time – the time taken between peak rainfall and peak discharge
Rising Limb – shows the increase in discharge on a hydrograph
Falling Limb – shows the return of discharge to normal / base flow on a hydrograph
Peak Rainfall – maximum rainfall (mm)
Peak Discharge – maximum discharge (cumecs)

Guide to interpreting hydrographs

Factors affecting the shape of hydrographs:

  • Impermeable rocks (e.g. granite) and soil (e.g. clay) will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly, this results in a short lag time. Permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing surface runoff, this results in a longer lag time. Porous rock allow water to be stored in them or pass through them and pervious rock allows water to flow along faults and bedding planes.
  • A drainage basin with a steep gradient will result in faster surface runoff and a shorter lag time than where the gradient is less steep allowing more time for infiltration to occur.
  • Heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff (short lag time). Slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow (longer lag time).
  • Previous weather conditions: prolonged rainfall may mean the soil has already been saturated by heavy rain. Its infiltration capacity will be reduced and more rain will flow as surface runoff. If the soil is dry it will be able to absorb more water during infiltration and therefore the lag time will be longer. If the ground surface is frozen lag time is short as water cannot infiltrates and passes quickly to the river as runoff
  • Impermeable man made surfaces such as concrete and tarmac are impermeable therefore rivers in urban drainage basins tend to have short lag times due to higher amounts of surface runoff and drainage systems taking water to rivers quickly.
  • Vegetated areas help to reduce flood risk by increasing the time it takes for water to reach a river (longer lag time) by encouraging infiltration (roots opening up the soil), intercepting water by their leaves and taking up water in their roots. Areas cleared by deforestation will respond quickly to rainfall due to the reduced interception
  • Large drainage basin – water will take longer to reach the river (long lag time). Small drainage basin – water will enter the river quicker (short lag time)
  • The presence of a dam will allow flow to be controlled, reducing flood risk and allowing rivers to gradually respond to heavy rainfall in a controlled way;