Because hydrological work is specially designed to deal with floods. As long as we find out many characteristics of a river, such as the average runoff for many years, the maximum peak flow in history, basin area, runoff coefficient, mean value and variation coefficient, we can predict the scale of a flood that will happen in this river, such as where the water level will reach and so on. This is the hydrological forecasting work. Hydrological forecast is equivalent to information about flood. With this information, we can accurately deal with future floods.