In the 13th century, a fishermen's village developed at the place where the river Amstel entered the IJ estuary. Later on, to prevent floods, dikes were built to protect the coast south of the IJ, and a dam with sluices was built in the river Amstel. This dam of the River Amstel gave the name to this fishermen's village. Even after it developed into a town, it still was called after the dam in the River Amstel.

Amstel + dam = Amsteldam or Amsterdam