Gaṅgā Sahasranāma of the Kāśī Khaṇḍa

The Gaṅgā Sahasranāma is the thousand names of Devī Gaṅgā, the river. Skanda tells it to Agastya in chapter 29 of the Kāśī Khaṇḍa of the Skanda Mahā Purāṇa. It comes after two chapters (27 and 28) in praise of Gaṅgā, in which Bhagavān Śiva teaches Bhagavān Vishṇu how great Gaṅgā is, as a form of his own Śakti. The Sahasranāma is the high point of that teaching.

The setting: the greatness of Gaṅgā (chapters 27 and 28)

The Sahasranāma is part of a longer teaching on how great Gaṅgā is. In chapters 27 and 28, Bhagavān Śiva teaches Bhagavān Vishṇu about her nature and power.

Gaṅgā as Śiva’s own Śakti

Bhagavān Śiva says: “She is another form of mine in a watery form. She is the very ātmā of welfare. She is the greatest Prakṛti, the mainstay of many brahmāṇḍas (cosmic eggs). She is the personified form of pure learning and compassion. She has three śaktis: those of will, knowledge and action.”

Gaṅgā in the four yugas

Kṛtayuga: holy tīrthas are everywhere, and all places are sacred.
Tretāyuga: Pushkara is the greatest tīrtha.
Dvāparayuga: Kurukshetra is the supreme tīrtha.
Kaliyuga: Gaṅgā alone is the greatest tīrtha.

Gaṅgā and moksha

Bhagavān Śiva says: “In Kaliyuga there is no other means of moksha than Gaṅgā for people of unclean minds. In Kṛtayuga, meditation alone; in Tretā, meditation and tapas; in Dvāpara, meditation, tapas and yajñas; but in Kaliyuga it is Gaṅgā alone.”

The power of her name

“Alakshmī (lack of wealth), Kālakarṇī (a female rākshasī), bad dreams and evil thoughts have no power over the devotee who does japa such as ‘Gaṅgā, Gaṅgā.'”

All tīrthas are within Gaṅgā

“All the holy rivers and holy places in all three worlds; all the dharmas; yajñas together with their gifts; all kinds of tapas; the four Vedas with their aṅgas; Vishṇu, Śiva, Brahmā and all the groups of devas; all the purushārthas: all these are present in Gaṅgā in subtle form.”

The name Vārāṇasī

The city got the name Vārāṇasī from the rivers Varaṇā and Asī, which are tributaries of Gaṅgā. This joins the supreme tīrtha-city to the identity of Gaṅgā herself.

The Sahasranāma (chapter 29)

The Gaṅgā Sahasranāma holds about a thousand names of Gaṅgā. Skanda tells them to Agastya. The names speak to Gaṅgā in these ways.

In her cosmic forms. Examples are Vishṇupādī, she who flows from the feet of Bhagavān Vishṇu, and Tripathaggāminī, she who flows in three paths, as the Gaṅgā of the sky, of the earth and of the underworld.
In her work of purifying. Examples are Pāpanāśinī, the destroyer of pāpa, and Tāraṇī, the one who takes people across and gives moksha.
In her divine identity, as a form of the Śakti of Bhagavān Śiva and as the supreme Prakṛti.
In her auspicious qualities that protect her devotees.
In her bond with the great tīrthas that lie along her banks.

The Sahasranāma is one of the longest single texts in the Pūrvārdha of the Kāśī Khaṇḍa, and it fills the whole of chapter 29.

The phalaśruti

Those who listen to the greatness of Gaṅgā, or recite it, including the Sahasranāma, are freed from great pāpa. The evil-minded, the doubters and the rationalists who take Gaṅgā for an ordinary river are called fools. But even those who bathe in Gaṅgā by chance, for a fee, or while busy with trade, reach svarga. The teaching is that Gaṅgā, like fire, purifies even those who touch her unwillingly.

Sources

Skanda Mahā Purāṇa, Kāśī Khaṇḍa, Chapter 29 (the Sahasranāma)
Skanda Mahā Purāṇa, Kāśī Khaṇḍa, Chapters 27 and 28 (the greatness of Gaṅgā)