The Krupa river is the Neretva left tributary and the main water current of Hutovo Blato, which leads the waters from Gornje Blato and Svitavsko lake into the Neretva river. The length of Krupa is 9 km with an average depth of 5 meters. Krupa does not have an actual source, but is actually an arm of Deransko lake. Also, the Krupa is a unique river in Europe, because the river flows both ways. It flows 'normally' from the 'source' to the mouth and from the mouth to the 'source'. This happens when, due to high water level and large quantity of water, river Neretva pushes the Krupa river in opposite direction .