The Martwa Wisla is a river, one of the branches of Vistula. It flows in Poland to Gdansk Bay.
Map of Martwa Wisla part 1 (http://gis.gdansk.pl/dziel/d-24.htm)
Map of Martwa Wisla part 2 (http://gis.gdansk.pl/dziel/d-30.htm)
Map of Martwa Wisla part 3 (http://gis.gdansk.pl/dziel/d-22.htm)
