The Kossau is a stream in the district of Plön in eastern Schleswig-Holstein, Germany. It drains the Tresdorf lake (Tresdorfer See) and flows past Lütjenburg before entering the Großen Binnensee. The stream was dammed by Rantzau Castle in the Middle Ages to form a lake which existed until the 18th century. It is about 22 kilometres (14 mi) long and most of its course consists of meanders. It is largely in a natural state and, together with its wet meadows, forms a rich habitat.