The Karasu (Turkish for 'black water') or Western Euphrates is a long river in eastern Turkey, one of the two sources of the Euphrates. It has a length of about 450 km. To the ancient Greeks the river was known as the Τηλεβόας (Telebóas).
The river rises on the Dumlu Dağ in Erzurum Province, and drains the plains around the city of Erzurum. It is joined by the Serçeme River, then flows west through Erzincan Province, turning south, then west and receiving the tributary Tuzla Su. Between Erzincan and Kemah it is joined by the Gönye River and passes through a rocky gorge. Near the small town of Kemaliye it receives its last tributary, the Çaltısuyu, before turning sharply southeast to flow through a deep canyon into the Keban Dam Lake on the Euphrates. Before construction of the Keban Dam the Karasu joined the Murat River 10 km above the dam site and 13 km above the town of Keban.
In 1996, 15 Turkish soldiers were killed when their personnel carrier fell into the river during a routine crossing.
When temperatures in Turkey dropped below -25 °C in 2005 the river froze over and was used as a football pitch by locals.