Stockholm has since its founding been a lock sitting between Lake Mälaren and the Baltic Sea, collecting toll from anyone passing through and generally not producing anything of value.
Back in the 17th century for example, a toll was levied on any products[a] being brought into the city for sale. To avoid the preddictable situation of huge markets being set up in what became the tax havens of the times, it was simultaneously made illegal to sell anything outside of the city.
Anyway, the city being a lock means that it must have some water flowing through it - and it does. Simply called "strömmen" or "the stream", it is what gives Stockholm its beautiful water mirror.