Bengawan Solo River (alternatively, Solo River, with Bengawan being an Old Javanese word for river) is the longest river in the Indonesian island of Java, it is approximately 600 km in length. Apart from its importance as a watercourse to the inhabitants and farmlands of the eastern and northern parts of the island, it is a renowned region in paleoanthropology circles. Many discoveries of early hominid remains have been made at several sites in its valleys, especially at Sangiran, including that of the first early human fossil found outside of Europe, the so-called "Java Man" skull. Bengawan Solo was the crash site of Garuda Indonesia Flight 421.
The Solo River (or Bengawan Solo) has its source by the volcano of Mount Lawu, on the border between Central Java and East Java. It passes through the major city of Surakarta (called Solo by the local inhabitants). An important early tributary to the Solo river is the Dengkeng River, which has its source in Mount Merapi. After passing through Solo, the river flows northward around Mount Lawu, and then turn eastward into East Java in the Ngawi regency.
After Ngawi the river turns northward again, forming the boundary between Blora Regency of Central Java and Bojonegoro regency of East Java. From the town of Cepu in Blora, the river turns eastward and passes through Bojonegoro regency's capital city. From there, it continues eastward through the Lamongan and Gresik regencies. The last part of the river's basin (roughly starting from Bojonegoro regency) is mostly flat land.