The Sarayu (also Sarju; Dev. सरयु saráyu- f., later Dev. सरयू sarayū-) is a river that flows through the Indianstates of Uttarakhand and Uttar Pradesh. This river is of ancient significance, finding mentions in the Vedas and the Ramayana. The Sarayu forms at the confluence of the Karnali (or Ghaghara) and Mahakali (or Sharda) in Bahraich District. The Mahakali or Sharda forms the Indian-Nepalese border. Ayodhya is situated on the banks of river Sarayu. Some mapmakers consider the Sarayu to be just a section of the lower Ghaghara River.
The river is mentioned three times in the Rigveda. The banks of the Sarayu are the location of the slaying of two Aryas at the hands of Indra in RV 4.30.18. It is listed together with western tributaries to the Indus: Rasā, Anitabha, Kubha, Krumu, and the Sindhu itself as obstacles crossed by the Maruts in RV 5.53.9. In this verse, Purisini appears as its epithet. At this stage of the earlier Rigveda, it apparently was a river west of the Indus system that corresponds to Iranian Harayu (Avestan acc. Harōiium, Old Persian Haraiva, modern Harē), the Herat river. It is invoked together with Sindhu and Sarasvati (two of the most prominent Rigvedic rivers) in the late hymn RV 10.64.
Later on, according to Ramayana 1.5.6, the Sarayu flowed beside the ancient city of Ayodhya, which is in the modern day Uttar Pradesh. It was a tributary of the Gogra. This Sarayu played a vital role for the city and life of Ayodhya, and according to the great Sanskrit epic Ramayana, is where Rama, the seventh Avatar of Vishnu immersed himself to return to his eternal, real Mahavishnu form, when he retired from the throne of Kosala. His brothers Bharata and Shatrughna also joined him, as did many devoted followers. The Sarayu is also the river upon whose banks King Rama was born.
According to a sub-story within the Ramayana, the banks of the River Sarayu is also the place where King Dashratha accidentally killed Shravan Kumar.
It is also called the only river flowing beneath the earth.
In ancient times Aciravati was one of the tributaries of the Sarayu.
While the Sarayu river plays an important role in RigVeda and Atharva Veda along with Ramayana. Ayodhya, capital of King Rama is mentioned on the banks of Sarayu river. Sarayu river is mentioned in great details in Ramayana. Ayodhya still exists on the banks of river Sarayu in Indian state of Uttar Pradesh. It is a tributary of the Ganges which meets two other rivers at the confluence (sangam) in Uttar Pradesh.