In Lake Constance on average several times a year underflows occur due to flood events in River Rhine. A hydrodynamic model was set up to analyze the conditions under which underflows form as well as their effect on deep water renewal. Observations of the water temperatures at the lake bottom during the flood event of August 2005, where River Rhine reached a discharge of 2200 m³/s, which is slightly below the discharge with 100-year return period, were used to calibrate the model. The model was successfully used to analyze several further, smaller underflows. In a model study of the flood event of June 2012, the influence of underflows to deep water renewal was investigated. By using different tracers to mark the inflowing water as well as the epilimnic water it was shown, that a large percentage of the epilimnic water is entrained into the underflow and therefore enhances deep water renewal. Several artificial modelling scenarios were set up, showing that primarily the concentrations of suspended solids determine the occurrence and extent of an underflow, while water temperatures are having a relatively small influence.