The Grand Canal probably follows the course of an ancient river (possibly a branch of the Brenta) flowing into the lagoon. Adriatic Veneti groups already lived beside the formerly-named "Rio Businiacus" before the Roman age. They lived in stilt houses and on fishing and commerce (mainly salt). Under the rule of the Roman empire and later of the Byzantine empire the lagoon became populated and important, and in the early 9th century the doge moved his seat from Malamocco to the safer "Rivoaltus".
Increasing trade followed the doge and found in the deep Grand Canal a safe and ship accessible canal-port. Drainage reveals that the city became more compact over time: at that time the Canal was wider and flowed between small, tide-subjected islands connected by wooden bridges...