Titus Livius (Livy), The History of Rome, Book 21
Rev. Canon Roberts, Ed.

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This was the first battle with Hannibal, and the result made it quite clear that the Carthaginian was superior in his cavalry, and consequently that the open plains which stretch from the Po to the Alps were not a suitable battlefield for the Romans. [2] The next night accordingly, the soldiers were ordered to collect their baggage in silence, the army moved away from the Ticinus and marched rapidly to the Po, which they crossed by the pontoon bridge which was still intact, in perfect order and without any molestation by the enemy. [3] They reached Placentia before Hannibal knew for certain that they had left the Ticinus; however, he succeeded in capturing some 600, who were loitering on his side of the Po, and were slowly unfastening the end of the bridge. He was unable to use the bridge for crossing, as the ends had been unfastened and the whole was floating down-stream. [4] According to Caelius, Mago with the cavalry and Spanish infantry at once swam across, whilst Hannibal himself took his army across higher up the river where it was fordable, the elephants being stationed in a row from bank to bank to break the force of the current. [5] Those who know the river will hardly believe this for it is highly improbable that the cavalry could have stood against so violent a river without damage to their horses and arms, even supposing that the Spaniards had been carried across by their inflated skins, and it would have required a march of many days to find a ford in the Po where an army loaded with baggage could be taken across. [6] I attach greater weight to those authorities who state that it took them at least two days to find a spot where they could throw a bridge over the river, and that it was there that Mago's cavalry and the Spanish light infantry crossed. [7] Whilst Hannibal was waiting near the river to give audience to deputations from the Gauls, he sent his heavy infantry across, and during this interval Mago and his cavalry advanced a day's march from the river in the direction of the enemy at Placentia. [8] A few days later Hannibal entrenched himself in a position six miles from Placentia, and the next day he drew out his army in battle order in full view of the enemy and gave him the opportunity of fighting.