Scholars pay growing attention to historical flood events and as part of these studies new source evidence has been integrated in research projects. This paper discusses the flooding of the River Rába/Raab (W-Hungary, E-Austria) in the first half of the seventeenth century as reflected by a previously understudied group of evidence: private letters. Through the example of the manor of Körmend, a market town by the River Rába from where within a relatively short period of time (1600–1658) more than 3,000 private letters were sent to the members of the Batthyány family, this topic and data set will be discussed. The paper aims to analyse the frequency and the seasonality of the floods on the one hand, and on the other to demonstrate the research potential of private letters in environmental historical investigations.