sa kadacit sarasvatya
upasprsya jalam sucih
vivikta eka asina
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Once upon a time he [Vyasadeva], as the sun rose, took his morning ablution in the waters of the Sarasvati and sat alone to concentrate.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The river Sarasvati is flowing in the Badarikasrama area of the Himalayas. So the place indicated here is Samyaprasa in Badarikasrama, where Sri Vyasadeva is residing.