The Gaṅgā-Sarayū Saṅgama is the holy meeting place where the Sarayū joins the great Gaṅgā. The Sarayū is the holy river of Ayodhyā and of the Sūryavaṃśa (the solar dynasty).

The Vālmīki Rāmāyaṇa, Bāla Kāṇḍa, describes this confluence as a holy place passed by Rāma and Lakshmaṇa in the company of Viśvāmitra at the start of their first great journey from Ayodhyā. It is held to be a puṇya-tīrtha (a pilgrimage place of great merit).

In the Vālmīki Rāmāyaṇa, Bāla Kāṇḍa

At the start of Rāma’s first journey with Viśvāmitra, the ṛshi led Rāma and Lakshmaṇa from Ayodhyā southward toward the river Sarayū. They crossed the Sarayū and went on to the region where the Sarayū joins the Gaṅgā.

The Bāla Kāṇḍa describes this area as full of the sounds of the forest, visited by ṛshis and ascetics, and made holy by its place at the meeting of two great sacred rivers. Viśvāmitra pointed out the beauty and holy meaning of the land as they passed through.

The Vālmīki Rāmāyaṇa, Bāla Kāṇḍa, Chapter 22, tells this.

Gopratāra and the end of Rāma’s avatāra

The confluence is also closely linked with the last episode of Rāma’s avatāra (descent). The Skanda Mahā Purāṇa, Ayodhyā Māhātmya, places Gopratāra, the supreme tīrtha of Ayodhyā, at the meeting of the Sarayū with the branch of the Gaṅgā called Gharghara. There Rāma made his mahāprasthāna (great departure) into the sacred waters at the end of his reign.

Sacred meaning

The meeting of any two great rivers is held to be supremely purifying in the śāstras. The Gaṅgā-Sarayū Saṅgama brings together the Vishṇupādī Gaṅgā and the Sarayū, which arose from Lake Mānasa and flowed through Ayodhyā. It is the meeting of two rivers of great sacred meaning.

Both rivers are linked with moksha (liberation). The Sarayū is linked in particular with the solar dynasty and with Rāmarājya (the rule of Rāma). The Gaṅgā is linked with the freeing of all who bathe in her.

Sources

Vālmīki Rāmāyaṇa, Bāla Kāṇḍa, Chapter 22
Skanda Mahā Purāṇa, Ayodhyā Māhātmya