The Vidrus fluvius, Greek Ouidros potamos, is a river of Ptolemy's Geographia (2.11) located in Germania.
Both the Vechte and the IJssel flowed to the IJsselmeer, formerly the Zuiderzee, which one Roman source (Pomponius Mela) called Flevus Lacus.
Many scholars (including Smith, Van der Aa, Bruijnesteijn and Anthon) equate the Vecht and the Vidrus.
Etymologically Vidrus is close to Indo-European *wod-or, which becomes Germanic *watar by Grimm's Law.
A derivation from "water" is consistent with Ortelius' statement that parts of Holland were called "Waterland.