A river hydrograph is a graph that shows the change in river level (or river discharge) over time. A bar graph of rainfall is sometimes also shown, in order to show the 'lag time' (the delay between peak rainfall intensity and peak discharge).
Different river catchments produce different shapes of hydrograph. A 'flashy' hydrograph has a short lag time, high peak discharge, and steep rising and falling limbs.In a flashy hydrograph, the river is more likely to flood.
A hydrograph with a longer lag time means the river is less likely to flood.