A river regime is the difference in the discharge of the river throughout the year. The information can easily be shown on a graph.
During the year in Britain, it would be expected that the discharge of most rivers would be greater in winter months than in the summer. River regimes will also reflect the area that they are in.
For example: The regime of an Alpine stream, has a minimal discharge compared to that of a British stream during the winter. This is because most of the precipitation in the Alps during the winter falls as snow. During the spring months, when the snow melted, the discharge of the Alpine river shows an incredible increase, which often leads to flooding problems. In contrast a British river wil slowly reduces its discharge throughout the drier summer months, until the autumn.
Flooding occurs due to a sudden increase in the amount of water travelling down a river, and can occur for a number of reasons:
- A rapid snow melt
- A prolonged period of heavy rain
- A sudden and intense fall of rain
- Human interventions, such as deforestation, which cause the water to run-off faster than previously.
The discharge of a river is shown on a graph called a flood or storm hydrograph. It shows the rainfall amount and then the discharge of the river. 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. Some, with steep slopes and little vegetation, will rise very quickly, and can be described as a "flashy river". These are the most likely to flood.
Those drainage basins with shallower slopes, and greater vegetation cover will infiltrate the water more, and release it at a slower rate into the river. This means there is less chance of flooding occurring.
Flood hydrographs are very important in predicting how a certain river will behave in a time of intense rainfall. The information could then also be used to plan the most appropriate form of flood prevention scheme for that particular river.