The Vistula is the longest and the most important river in Poland, at 1,047 km (651 miles) in length. The watershed area of the Vistula is 194,424 km² (75,067 square miles), of which 168,699 km² (65,135 sq. miles) lies within Poland (splitting the country in half).

Source: wikipedia.org (http://en.wikipedia.org/wiki/Vistula)