The Lagoon: Origins, Environment, and Interventions
"Venice is inconceivable without its lagoon; it would not, could not, exist without its lagoon." - UNESCO RAMSAR Report, 2003 
During the height of the Pliocene Age, more than a million years ago, the waters of the Adriatic Sea, which stretched as far inland as Turin, receded, so far in fact, that the islands on which Venice rests today, became part of the mainland. Gradually, a lagoon of about 210 square miles was formed at the intersection of three rivers: the Brenta, the Piave, and the Sile, and the Adriatic. Throughout this area were scattered islands and mudflats, as well as a complicated system of natural channels. As a generally shallow area, the lagoon met the Adriatic at three main breaks, the Lido, Malamocco, and the Chioggia channels, in the 25-mile long bar of narrow land that runs the length of the seaward side.
Venice was founded in the fifth century as a haven for persons fleeing from the barbarians. By the fourteenth century the cluster of 118 islands had become a single entity connected by bridges and canals. As a wealthy mercantile city, Venice depended heavily on the surrounding water for transportation, food, and protection, boasting that the marshy waters were ‘too shallow for invading ships, too deep for marching armies.’As Venice became an increasingly prominent center for trade, the need for deeper navigational channels grew, sparking a series of modifications made to the lagoon. Beginning in the fifteenth century, seven river mouths that directly fed into the lagoon were diverted to the north and south of Venice. In addition to the river diversions, construction began on great coastal defenses. Massive stonewalls were built running the length of the seaward coast of the lagoon.
Fletcher, Caroline and Jane Da Mosto. The Science of Saving Venice. Turin: Umberto Allemandi & Co., 2004. p. 17.
Fay, Stephen, and Phillip Knightley. The Death of Venice. New York: Praeger Publishers, 1976. p. 33.