Titus Livius (Livy), The History of Rome, Book 21
Benjamin Oliver Foster, Ph.D., Ed.

31. After encouraging them with this exhortation, he bade them refresh themselves and make ready for the march. [2] Setting out the following day he advanced up the Rhone towards the interior of Gaul, not that it was the more direct way to the Alps, but believing that the farther he retired from the sea, the less likely he was to fall in with the Romans, [3??] with whom he had no mind to fight a battle until he should arrive in Italy. The fourth day's march brought him to the Island. [4] There the rivers Isara and Rhone, rushing down from different Alps, unite their waters, after enclosing a considerable territory, and the Island is the name which has been given to the plains lying between them. [5] Near by is the country of the Allobroges, a tribe, even at [p. 91]that early day, inferior to no Gallic tribe in wealth¹ or reputation. Just then it was a prey to discord.² [6] Two brothers were disputing the sovereignty. The elder, Braneus by name, who had held sway before, was being driven out by a faction of juniors headed by the younger brother, whose right was less but his might greater. [7] This quarrel having very opportunely been referred to Hannibal for settlement, who thus became arbiter of the kingdom, he espoused the sentiments of the senate and the leading men and restored the sovereign power to the elder. [8] In requital of this service he was assisted with provisions and supplies of every sort, particularly clothing, which the notorious cold of the Alps made it necessary to provide.

[9] Having settled the contentions of the Allobroges, Hannibal was now ready for the Alps; but instead of marching directly towards them, he turned to the left,³ to the country of the Tricastini, and thence proceeded through the outer borders of the territory of the Vocontii to the Tricorii, by a road which nowhere presented any difficulties, until he came to the Druentia.⁴ This, too, is an Alpine river and by far the most difficult of all the rivers of Gaul [11] to [p. 93]cross; for, though it brings down a vast volume of⁵ water, it does not admit of navigation, since, not being confined within any banks, but flowing at once in several channels, not always the same, it is ever forming new shallows and new pools —a fact which makes it dangerous for foot-passengers as well —besides which it rolls down jagged stones and affords no sure or stable footing to one who [12] enters it. And at that time, as it happened, it was swollen with rains, and the crossing took place amidst the wildest tumult, for the men —besides their other difficulties —were confused by their own excitement and bewildered outcries.