The river Fli (or Flehi) divided the northern part of the Netherlands then called Friesland into a western and eastern part and was used as a connection from the inlands out to the North Sea. In the 11th century, after heavy rainfalls, the river flooded and set a large part of the land under water and not long after the Zuider Zee was formed.
Since the 13th century West-Friesland is protected by the Westfriese Omringdijk, a dike that was built to protect the land against the salty water of the Zuider Zee. This dike currently still protects the eastern part of West-Friesland against the IJsselmeer.
For about 300 years West-Friesland operated as an autonomous area, because the Westfrisians didn't want to subject to the Dutch authorities. Floris V, the Count of Holland, tried hard during his reign, but finally his successor John II defeated the Westfrisians in 1297 and thus they lost their independence. However even tough it formed one province with Holland in the Dutch Republic, it was recognized as another region and the parliament of that province, commonly known as Holland, was formally known as the States of Holland and Westfriesland, showing that West-Friesland was still recognized in its own right.