Matsya Purana — Cosmography of Śākadvīpa and Successive Dvīpas: Mountains
तेषु नद्यश्च सप्तैव प्रतिवर्षं समुद्रगाः द्विनाम्ना चैव ताः सर्वा गङ्गाः सप्तविधाः स्मृताः //
teṣu nadyaśca saptaiva prativarṣaṃ samudragāḥ dvināmnā caiva tāḥ sarvā gaṅgāḥ saptavidhāḥ smṛtāḥ //
Among them there are precisely seven rivers that, in each region, flow to the ocean. All bear two names, and they are remembered as Gaṅgā in seven distinct forms.
It does not describe Pralaya directly; it maps sacred geography by stating that seven major ocean-bound rivers in the world’s regions are collectively regarded as seven forms of Gaṅgā.
By identifying Gaṅgā as a sevenfold sacred presence, it supports dharmic practice through tīrtha-recognition—encouraging rulers and householders to protect rivers and to undertake disciplined pilgrimage and ritual bathing where appropriate.
The ritual takeaway is the elevation of multiple river systems as Gaṅgā-equivalents, implying that snāna (sacred bathing), offerings, and tīrtha rites may be performed at these rivers with Gaṅgā-like merit in regional contexts.