There is evidence that, towards the close of the mediaeval period, great storms and tidal inundations occurred on the shores of the North Sea and Baltic, and in the course of these floods, culminating in 1297, the Zuider Zee was formed from a lake that existed in its neighbourhood, by the breaking down of dykes.
The security of the Zuider Zee for trade and fishing purposes was the first factor in the commercial development of North Holland, and the cities of Medemblik, Enkhuizen, Hoorn, Edam and Monnikendam, though now little more than market centres for the surrounding district, possessed a large foreign commerce in the 16th and 17th centuries.
In the Zuider Zee small herring, flat fish, anchovies and shrimps are caught, the chief fishing centres being the islands of Texel, Urk and Wieringen, and the coast towns of Helder, Bunschoten, Huizen, Enkhuizen, Vollendam, Kampen, Harderwyk, Vollenhove.
Groningen communicates with the Lauwers Zee by the Reitdiep (1873-1876), while the canal to Winschoten and the Stadskanaal, or State canal (1877-1880), bring it into connexion with the flourishing fen colonies in the east of the province and in Drente.
Bunschoten on the Zuider Zee is a fishing village; Venendaal, on the south-eastern border, originally a fen-colony, is now a market for the bee-keeping industry in the east.
In 1439 it decreed that no one might trade in all the district between the Ems and the Lauwers Zee except burghers, and those who had purchased the burwal (right of residence in the city) and the freedom of the gilds.
From the mainland, and stretching from the Zuider Zee E.
Besides the Reit Diep, there are the Ems Canal and the Damster Diep, connecting it with Delfzyl and the Dollart, the Kolonel's Diep with Leeuwarden, the Nord Willem's Canal with Assen and the south and the Stads-Canal south-east with the Ems. Hence steamers ply in all directions, and there is a regular service to Emden and the island of Borkum via Delfzyl, and via the Lauwers Zee to the island of Schiermonnikoog.
Below Nyack the river expands into Tappan Zee or Bay, which is about 3 m.
There is reason to believe that the anchovies found at the western end of the English Channel in November and December are those which annually migrate from the Zuider Zee and Scheldt in autumn, returning thither in the following spring; they must be held to form an isolated stock, for none come up from the south in summer to occupy the English Channel, though the species is resident on the coast of Portugal.