Maglekilden is a small underground spring which emerges behind Roskilde Cathedral. Its water flow is 10-20 l/s.
Previously the water was simply diverted into the sewer system and from there into Roskilde harbour. At one point, someone in the municipality came up with the idea of using the water to create a small recreational watercourse flowing through an artificial lake created for this purpose in a nearby park named Byparken.
The watercourse was designed for a flow of roughly 15 l/s. To safeguard against flooding in the park in the event that the spring produced more than 15 l/s, a flow regulator was installed at the point where the spring emerged. It ensures that only 15 l/s is diverted to the watercourse. Any water volume exceeding 15 l/s is diverted into the harbour.
The set-up was officially opened with great festivities in 1998 and still works today, to the deep satisfaction of the municipality, park users and many duck families.