The total area of the Baltic Sea drainage basin (Fig. A1) is 1,745,136 km², and the annual inflow of freshwater with the rivers is 475 km³ yr^-1, which corresponds to 15,130 m³ s^-1 . In Sweden and Finland, there are several medium-sized rivers, whereas a few large rivers dominate in the southern and eastern parts of the Baltic Sea drainage basin. Together, the ten largest river basins (Neva, Vistula, Daugava, Neman, Oder, Kemijoki, Göta, Ångermanälven, Lule, Indalsälven) cover 56% of the total drainage area and contribute 52% of the total freshwater input to the Baltic Sea (Table A2). It is also noteworthy that the five largest lakes in Europe (Ladoga, Onega, Vänern, Peipus and Vättern) are all located in the drainage basin of the Baltic Sea.

There is a large spatial variation in specific runoff: the highest values (i.e. up to 15 L s^-1 km²) have been recorded the mountain regions of northwestern Sweden and the lowest values (approximately 5 L s^-1 km²) in the Polish and German parts of the drainage basin. A majority of the rivers normally have a pronounced flow peak during snowmelt in spring. This is particularly true for the rivers in northern Sweden and Finland and the major rivers in the eastern parts of the drainage basin. Dams constructed to level out seasonal variation in riverflow are primarily found in northern and central Sweden, and the lowest level in regualtion capacity is found in four of the ten largest rivers in the area, i.e. the Daugava, Neman, Vistula and Oder Rivers. Further details regarding hydrology can be found in an article by Bergström and Carlsson [41].

Considered in its entirety, the Baltic Sea drainage area is dominated mainly by forests and to a somewhat lesser extent by arable land (Fig. A2), although there are substantial differences among sub-basins. In the drainage basin of the Gulf of Bothnia, for example, arable land and pasture accounts for about 5% of the area, whereas the same land-cover categories make up about 50% of the drainage basin of the Baltic proper. Poland alone accounts for about 40% of the arable land in the whole Baltic Sea drainage basin. The bedrock in the Nordic countries consists primarily of species of granites and gneisses, whereas sedimentary bedrocks dominate in the southern and eastern part of the drainage basin. The Baltic Sea drainage basin has about 85 million inhabitants; the highest population density is found in the southern parts of the basin, and Poland alone accounts for about 45% of the total population. Most of the largest cities are located on the coast, and the largest inland cities are located in the Neman, Vistula and Oder Basins.

Rivers play a crucial role in the total input of nutrients to the Baltic Sea. Several studies have indicated that nutrient delivery along this route may exceed the combined contribution from atmospheric deposition on the sea, point emissions along the coast, and nitrogen fixation by marine organisms. There are also strong indications that the riverine loads of nutrients have increased dramatically over the past century. However, data regarding the historical development of nutrient loading of the Baltic Sea are uncertain, and even loading during the past few years has been a matter of controversy.