Strabo Geography 4.3.3

[3] The first of all the nations dwelling on the Rhine are the Helvetii, amongst whom are the sources of that river in Mount Adula, ¹ which forms part of the Alps. From this mountain, but in an opposite direction, likewise proceeds the Adda, which flows towards Cisalpine Gaul, and fills lake Larius, ² near to which stands [the city of] Como; thence it discharges itself into the Po, of which we shall speak afterwards. The Rhine also flows into vast marshes and a great lake, ³ which borders on the Rhaeti and Vindelici, ⁴ who dwell partly in the Alps, and partly beyond the Alps. Asinius says that the length of this river is 6000 stadia, but such is not the case, for taken in a straight line it does not much exceed half that length, and 1000 stadia is quite sufficient to allow for its sinuosities. In fact this river is so rapid that it is difficult to throw bridges across it, although after its descent from the mountains it is borne the remainder of the way through level plains; now how could it maintain its rapidity and vehemence, if in addition to this level channel, we suppose it also to have long and frequent tortuosities? Asinius likewise asserts that this river has two mouths, and blames those who say that it has more. ⁵ This river and the Seine embrace within their tortuosities a certain extent of country, which however is not considerable. They both flow from south to north. Britain lies opposite to them; but nearest to the Rhine, from which you may see Kent, which is the most easterly part of the island. The Seine is a little further. It was here that divus Caesar established a dock-yard when he sailed to Britain. The navigable portion of the Seine, commencing from the point where they receive the merchandise from the Saone, is of greater extent than the [navigable portions] of the Loire and Garonne. From Lugdunum ⁶ to the Seine is [a distance of] 1000 stadia, and not twice this distance from the outlets of the Rhone to Lugdunum. They say that the Helvetii, ⁷ though rich in gold, nevertheless devoted themselves to pillage on beholding the wealth of the Cimbri, ⁸ [accumulated by that means;] and that two out of their three tribes perished entirely in their military expeditions. However, the multitude of descendants who sprang from this remainder was proved in their war with divus Caesar, in which about 400,000 of their number were destroyed; the 8000 who survived the war, being spared by the conqueror, that their country might not be left desert, a prey to the neighbouring Germans. ⁹