A hydrograph is a graph which plots the discharge of a river (the amount of water flowing through it) against time taken. It shows the amount of time between the start of the rain storm and when the river begins to fill up. The briefer the lag time and the higher the peak, the more likely a river is to flood.

Many people worry about flood Hydrographs as they look complex and difficult to understand but as long as you know the general facts and don't panic you should be OK in the GCSE Geography exam.