Nearly all of Poland is drained northward into the Baltic Sea by the Vistula, the Oder, and the tributaries of these two major rivers. About half the country is drained by the Vistula, which originates in the Tatra Mountains in far south-central Poland. The Vistula Basin includes most of the eastern half of the country and is drained by a system of rivers that mainly join the Vistula from the east. One of the tributaries, the Bug, defines 280 kilometers of Poland's eastern border with Ukraine and Belarus. The Oder and its major tributary, the Warta, form a basin that drains the western third of Poland into the bays north of Szczecin. The drainage effect on a large part of Polish terrain is weak, however, especially in the lake region and the inland areas to its south. The predominance of swampland, level terrain, and small, shallow lakes hinders large-scale movement of water. The rivers have two high-water periods per year. The first is caused by melting snow and ice dams in spring adding to the volume of lowland rivers; the second is caused by heavy rains in July.