Story of the disappearance of Saraswati River is found in the Anushasana Parva of the Mahabharata. Varuna, the God of seas, abducted Bhadra, wife of Sage Utathya. The sage was a descendant of Sage Angiras. The Sage asked the Devas to mediate with Varuna. However, Varuna refused to give back Bhadra.
An enraged Utathya undertook severe penance and meditation. By the power of his austerities, the sage dried up the waters of the world. He then asked Saraswati River to disappear into the sands and let the land deserted by her be impure.
Without water all living beings suffered. Finally, Varuna relented and returned Bhadra.
The sage then gave relief to living beings by returning water. This also entered the suffering of Varuna.
But Saraswati River did not appear again.