Greifswald is located in the northeast of Germany, approximately equidistant from Germany's two largest islands, Rügen and Usedom. The town is situated at the south end of the Bay of Greifswald, the historic centre being about up the river Ryck that crosses the town. The area around Greifswald is mainly flat, and hardly reaches more than 20 m above sea level. Two islands, Koos and Riems, are also part of Greifswald. Three of Germany's fourteen national parks can be reached by car in one hour or less from Greifswald.