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 (),Stephanus of Byzantium, Ethnika, A35.21 and was considered a river of ancient Thrace that flowed between Selymbria and Byzantium (the later Istanbul). Pliny the Elder also calls it Pydaras.
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