The Prosna is a river in central Poland, a left tributary of the Warta river (near Pyzdry), with a length of 227 kilometres and a basin area of 4,917 km2 (all in Poland).[1] Until 1918, it marked the westernmost border of the Russian Empire. The towns of Kalisz and Chocz were right on the old frontier. After the proclamation of the second Polish Republic it marked the border between interwar Germany and Poland.[2]