Variations of the runoff of the Rhine are a result of its alpine situation, its source areas and the precipitation in the watersheds of its tributaries.
Along the High Rhine and the southern Upper Rhine, floods mainly occur in summer, when alpine glaciers and snowfields melt and the winter precipitation is again released.
On the other hand, along the Middle and Lower Rhine, the great floods occur in winter and springtime. They are fed by melting snow in the midlands or rainwater which cannot infiltrate the frozen or water saturated soil. In both cases, water rapidly runs off at the surface and causes brooks and rivers to top their banks.
Cold winters – with precipitation mainly as snow and dry summers with little precipitation, e.g. that of 2003, may lead to low flow.