Matsya Purana — Kailasa
सप्तमी त्वनुगा तासां दक्षिणेन भगीरथम् तस्माद्भागीरथी सा वै प्रविष्टा दक्षिणोदधिम् //
saptamī tvanugā tāsāṃ dakṣiṇena bhagīratham tasmādbhāgīrathī sā vai praviṣṭā dakṣiṇodadhim //
But the seventh stream, following after them, went along the southern course of the Bhāgīratha; therefore that branch is indeed called Bhāgīrathī, having entered the Southern Ocean.
It does not describe Pralaya; it maps sacred geography, explaining how a particular branch of the Bhāgīratha/Gaṅgā is named Bhāgīrathī due to its southward course and ocean-entry.
Indirectly, it supports tirtha-yātrā and reverence for sacred waters: knowing river courses and their sanctity guides householders in pilgrimage, vows, and river-rites, and guides kings in protecting holy sites and waterways.
Ritually, it emphasizes the sanctity of a river’s sangama (ocean-confluence), a prime location for snāna, tarpaṇa, and śrāddha; architecturally, it implies the importance of siting ghāṭas and shrines near major sacred water junctions.