CC Madhya 18.222

ei-mata cali‘ prabhu ‘prayāga’ āilā
daśa-dina triveṇīte makara-snāna kailā
Word for word: 
ei-mata — in this way; cali’ — walking; prabhu — Śrī Caitanya Mahāprabhu; prayāga — the holy place named Prayāga; āilā — reached; daśa-dina — ten days; triveṇīte — at the confluence of the rivers Ganges and Yamunā; makara-snāna kailā — bathed during the festival of Makara, or Māgha-melā.
Translation: 
Śrī Caitanya Mahāprabhu finally arrived at Prayāga and for ten successive days bathed in the confluence of the rivers Yamunā and Ganges during the festival of Makara-saṅkrānti [Māgha-melā].
Purport: 

Actually the word triveṇī indicates the confluence of three rivers — namely the Ganges, Yamunā and Sarasvatī. Presently the Sarasvatī River is not visible, but the river Ganges and the river Yamunā merge at Allahabad.