Lake Mälaren is the third largest lake in Sweden (see Figure 1), with 24 cities and municipalities bordering the lakefront, including Stockholm. The lake is the drinking water source for two million people and also serves as a major port (see Figure 2).

The risk of flooding in Lake Mälaren is very high due to milder winters and heavier rainfalls. The winter of 2000 was unseasonably wet and much more water flowed into the lake than could be let out through the sluices that manage the water levels. There was extensive local surface water flooding and flooding nearly reached the central underground station tunnel of Stockholm.

Local disruption was thankfully a one-off at that point in time; however, it is forecast to be less of a one-off in a changed climate. Forecasts show that winter rainfall in Stockholm will increase, there will be greater year-on-year variations in precipitation, and the snowmelt will be greater. So there will be much more water flowing into the lake in the future.