A river regime is the term used to describe the annual variations in a specified river’s discharge.
A river’s discharge is the volume of water flowing through a river channel.
This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second.
Sometimes these measure/unit is known as cumecs.
A river’s regime is shown on a graph called a hydrograph.
A hydrograph shows the discharge of a river as well as total rainfall in the river’s basin/catchment area over a period of time, before, during and after the storm.
It allows for a relationship between the rainfall falling in a river’s catchment area and the river’s discharge.
Such information can be used to, for example, predict the risk of flooding in a given area after a storm event.
A storm hydrograph. Image credit weebly.com
As already said a river’s regime is shown on a storm hydrograph.
During a storm most of the rain falls onto the land rather than directly into the river.
The water then will make its way into the river and you can use a hydrograph to see how quickly this occurs.
By looking at the peak rainfall and comparing it with the peak discharge you can work out the lag-time (the time between the two peaks).
Different catchment areas will have different flood hydrographs.
A river’s regime is shown on an annual hydrograph with all the months listed.
Characteristics of a hydrograph
Peak discharge-shows the maximum amount of flow in the river.
Peak rainfall-the maximum amount of rainfall and when it fell.
Lag time-the difference between the peak rainfall and the peak discharge i.e the time it takes for the rain to reach the river.
A rising limp which shows a rise in discharge.
A falling limp which shows a fall in discharge.
Factors affecting a river’s regime.
Seasons- there will be a rise in discharge during summer/rain months and a fall in discharge in the dry months when there is little rainfall. There will be a lag time as water moves through the ground and from storages into the stream.
Climate-rivers that pass through Mediterranean climates tend to have more than one peak period as they have another surge in discharge during the winter months when these regions receive their rains. The same is also true of rivers that pass through areas that experience snow for example the Nile’s famed floods are due to snow melting in the Kilimanjaro mountains resulting in peak discharge even in the hot dry months as water moves from snow storage e.g. glaciers into the streams.
Geology for example rivers that flow through porous and pervious rocks tend to have smaller peaks/small changes in discharge as opposed to rivers in granite (non-porous and impervious) rocks.
Human activities for example urbanisation results in more impervious surfaces and very high peaks, short lag time and higher peaks (differences between the lowest and highest discharge.)