Before the St. Elizabeth's flood (1421) the Meuse flowed just south of today's line Merwede–Oude Maas to the North Sea and formed an archipelago-like estuary with the Waal and the Lek. This system of numerous bays, estuary-like extended rivers, many islands and constant changes of the coastline, is hard to imagine today. From 1421 to 1904, the Meuse and the Waal merged further upstream at Gorinchem to form the Merwede. For flood protection reasons, the Meuse was separated from the Waal through a lock and diverted into a new outlet called the "Bergse Maas", then the Amer flowing into the former bay known as the Hollands Diep.