Page "Elbe" Paragraph 6

At Mělník its stream is more than doubled in volume by the Vltava, or Moldau, a river which winds northwards through Bohemia.

Although upstream from the confluence Vltava is longer ( against ), has larger discharge and larger drainage basin, due to historical reasons ( at the confluence the Elbe flows through prominent wide valley while the Vltava, flowing into the valley, meets the Elbe at almost a right angle, so it appears as a tributary ) the river continues as Elbe.