Since 100 years the influence of forest coverage on the water cycle in general and on flood runoff formation in specific represents a basic question in the field of forest hydrology in Switzerland. On the 26th of December 1999 the storm Lothar caused great damage in Europa, including the Sperbelgraben catchment. This incident offered a unique opportunity to investigate the role of forests in preventing floods - especially considering different forest site types - with a nested approach in the Sperbelgraben. Theoretical considerations show that the effect of forest coverage on runoff formation is heavily dependent on the soil characteristics. The investigated forest site types contribute variably to the fast runoff formation, but there was little effect observed from the forest coverage itself.