Jakarta is located on the northwestern coast of Java Island, at the mouth of the Ciliwung River on Jakarta Bay, which is an inlet of the Java Sea. The northern part of Jakarta is constituted on a plain land, approximately eight meters above the sea level. This contributes to the frequent flooding. The southern parts of the city are hilly. There are about 13 rivers flowing through Jakarta, mostly flowing from the hilly southern parts of the city northwards towards the Java Sea. The most important river is the Ciliwung river, which divides the city into the western and eastern principality. The city border is the province of West Java on its east side and the province of Banten on its west side. The thousand islands, which is a part of the administrative region of Jakarta, is located in the Jakarta Bay. These 105 islands are located 45 kilometres (28 mi) on the north part of the city.