Hydrographs can be different shapes. The characteristics of the river and how likely it is to flood affects its shape.
A gentle hydrograph shows the river is at low risk of flooding. These types of hydrograph have a gentle rising limb and a long lag time which means it takes longer for the peak rainfall to reach the river channel, so the river discharge is increasing slowly.
A gentle hydrograph
Flashy hydrographs have a steep rising limb and a small lag time. This indicates the river discharge increases rapidly over a short period of time, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.
A flashy hydrograph