The Karasu (Turkish for 'black water') is a short river in the Istanbul metropolitan area; it flows into the lake formed by the Büyükçekmece Dam. Anciently, the river was known as the Athyras (Ancient Greek: Ἀθύρας),[1] and was considered a river of ancient Thrace that flowed between Selymbria and Byzantium (the later Istanbul).[2][3] Pliny the Elder also calls it Pydaras.[3]