Arresø is Denmark's largest lake, based on area. The lake runs into Roskilde Fjord through the Arresø Canal in Frederiksværk.

There are a number of rivers and streams running into the Arresø of which the Pøleå is the most significant. Especially along the lake's west side there are a number of hills and ridges. Among these is Maglehøj in the Frederiksværk and Arrenakke Hills, which offers a magnificent view over the lake.