Lycus or Lykos was an ancient river of Lydia

Lydia

Lydia was an Iron Age kingdom of western Asia Minor located generally east of ancient Ionia in the modern Turkish provinces of Manisa and inland İzmir. Its population spoke an Anatolian language known as Lydian....

 that flowed in a southwesterly direction by the town of Thyatira

Thyatira

Thyateira is the name of the modern Turkish city of Akhisar . The name comes from Koine Greek "Θυάτειρα" . The Turkish equivalent of Thyateira is Tepe Mezarligi. It lies in the far west of Turkey, south of Istanbul and almost due east of Athens...

. Whether it emptied itself directly into the Hermus

Hermus

In Greek mythology, Hermus is a name attributed to multiple characters.-River god:Hermus is the god of the river Hermus located in the Aegean region of Lydia . Like most of the river-gods, he is the son of Oceanus and Tethys...

, or only after joining with the Hyllus

Hyllus

In Greek mythology, Hyllus was the son of Heracles and Deianira, husband of Iole, nursed by Abia....

, is uncertain. (Plin.

Pliny the Elder

Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

v. 31; comp. Wheler, vol. i. p. 253; P. Lucas, Troisieme Voyage, vol. i. p. 139, who, however, confounds the Lycus with the Hermus.)