Why do poland's rivers flow north?

Poland is a plain region and it experiences northward slope, so river flows north side.

Explanation:

Poland is the one of the European country located in the 50 to 60 latitude which is in sub-polar region and its capital is Warsaw. Vistula and Oder are rivers flowed from south to northward in Poland. It experiences a northward slope because Baltic sea present in the north of the country, generally running water flows and depends on the topographical structure. Then the south of Poland surrounds many mountainous structures such as Carpathian Mountain and Moravian Mountain.  

answer biscay bay;