Why does water near the heads of Baltic Sea gulfs become nearly fresh in spring?

In the Baltic Sea, the water near the heads of its gulfs becomes nearly fresh in the spring primarily due to the influx of river runoff that increases significantly during the spring thaw.1

The Baltic region experiences a temperate continental climate, and its major rivers—such as the Vistula and the Oder—become swollen by snow meltwater during this season. Because the Baltic Sea is nearly landlocked and has a restricted exchange of water with the ocean, this large volume of fresh surface runoff significantly reduces the salinity of the water, particularly in the inner reaches and at the heads of gulfs like the Gulf of Bothnia.213