The focus of this article is on a curious episode at the end of the first book of Tacitus’ Annals. It is argued that Tacitus here is at his most metaphoric and allusive, allowing a senatorial debate on the possibly prophetic meaning of an inundation of the Tiber to become a debate about the overwhelming power of the river's namesake Tiberius. Parallels from Dio (and perhaps also from Livy) indicate that inundations of the Tiber by the end of the Republic had become prophetic warnings of the rise of the dynasts undermining the stability of the Republic. In Tacitus, procedural anomalies and suggestive wordplay bring to the fore the religious and constitutional issues that in the Senate's handling of this Tiberine prodigium reflect its submission to the ever more oppressive power of Tiberius.
