It is the first time evidence has been found from 1894 to back up other accounts from Passau residents that Hitler, who grew up in the town, nearly died there as a child.
In 1980, a priest called Max Tremmel told how his predecessor Johann Kuehberger had revealed to him that he rescued the terrified Hitler from the river.
Another resident, Anna Elisabeth Rosmus, recalled: "Everyone in Passau knew the story. Some of the other stories told about him were that he never learned to swim and needed glasses."
Years later, the Nazi leader told his generals how he played on the banks of the Inn, but he never revealed the drowning tale.