Usserød Å, the principal drainage of Sjælsø Lake.

Several watermills and early industrial sites are located along its course, the largest being Usserød Textile Mill.

Usserød Å extends from the south side of Sjælsø Lake.

Its first part is heavily regulated and runs through the outskirts of Birkerød, Eskemose Skov and the Istedrød area.

It then passes under the Helsingør Motorway and continues through the protected Usserød Ådal (Usserød Valley) at Mortenstrup before entering a suburban neighbourhood just north of Kokkedal station.