Kaystros (Καυστρος) was a River-God of Lydia in Anatolia (modern Turkey). The River Kaystros had its headwaters on Mount Tmolos and flowed into the Aegean Sea near Ephesos on the Lydian border with Karia. The next major rivers were the Meles and Hermos in the north, and Maiandros to the south.
Okeanos & Tethys
"And not far from Leimon [in Karia] is . . . a hero-temple of Kaystros and a certain Asios, and the Kaystros River that streams forth near by."
Strabo, Geography 14. 1. 44