Question

what was known a sapta sindhu

Answer 1

Rigvedic rivers
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Rivers, such as the Sapta Sindhavah ("seven rivers" Sanskrit: सप्त सिन्धव:)[1] play a prominent part in the hymns of the Rig Veda, and consequently in early Hindu religion. Vedic texts have a wide geographical horizon, speaking of oceans, rivers, mountains and deserts. “Eight summits of the Earth, three shore or desert regions, seven rivers.

Answer 2

Sapta Sindhu refers only to the sarasvati and it's own tributes