The IJ is a bay of the marker Sea. It forms the southernmost part of the Dutch inland lake, and adjacent to the polder De Nes, the Pampushaven, the Hollandse Brug and the mouth of the IJ. The IJ is a significant bird resting place.

Since 1998, several artificial islands for the Amsterdam IJburg development area were washed out. 2004 but this was stopped because the impact on nature were not sufficiently examined.

The IJ is bordered to the east by the Gooi- and Flevoland.