Most of the municipalities around Lake Mälaren take their drinking water from the lake. In order to prevent sea water from the Baltic entering Lake Mälaren and in order to save water, the water level of the lake is regulated by opening and closing various sluice gates in Stockholm and Södertälje.
It is the responsibility of the Port of Stockholm, on behalf of the City of Stockholm, to ensure that this regulation is carried out in accordance with the decisions of the riparian court. SMHI in Norrköping monitors that the decisions of the riparian courts are observed.
Lake Mälaren is regulated when the water level sinks below or rises to a certain level above the west lock sill in the Karl Johan Lock. The normal water level for Lake Mälaren is 4.14 metres above the lock sill.
When the water level is below 4.10 metres, all sluice gates and other draining points in Södertälje and Stockholm are closed. When the water level exceeds 4.10 metres, the sluice gate at Riksbron is opened. After that the various draining points are opened in this order: the Stallkanal gate, the gate in the draining channel by Karl Johan’s Square and lastly the gate in the Karl Johan Lock. If the water level is higher than 4.60 metres above the lock sill, draining is started at the locks in Hammarby and Södertälje.
If the water level in the Baltic is higher than that in Lake Mälaren, all gates have to be closed. Sea water must not enter Lake Mälaren och affect the fresh water intake at the Stockholm waterworks.
Strong currents can be dangerous and all boat traffic between Centralbron and Riksbron is prohibited. There are buoys west and east of Riksbron to mark the danger zone by the sluice gate. There are no buoys by Stallbron, but there are signs with the text ”Förbud mot båttrafik” (Boat traffic prohibited).
The Port of Stockholm also cooperates with Fritid Stockholm in order to look after the interests of anglers. When the gate at Riksbron is open, the currents become very strong to the east of the gate, which could be dangerous for anglers who are in the river channel. There are signs on the quay walls with warnings of the danger.