The Gaul the Franks took over was, and would long remain, a very Roman place. One had to go into cities to see the amphitheaters and temples which can still be found in some places. But the aqueducts could be seen from far off and were numerous enough that a traveler had a good chance of seeing some even without ever entering a city.
I'd known of a few aqueducts - those at Nimes, Lyons. But it turns out that numerous major cities had them: Frejus, Marseille, Aix, Arles, Beziers, Narbonne, Vienne, Valence, Geneva, Toulouse, Cahors, Bordeaux, Perigueux, Poitiers, Limoges, Rodez, Clermont-Ferrand, Bourges, Angers, Vichy, Sens, Meaux, Paris, Rouen, Bayeux, Le Mans, Rennes, Metz, Reims, Boulogne-sur-Mer, Soissons, Besancon...
What is more, there were also aqueducts at far lesser-known places: Sauvebonne, Antibes, Fos-Sur-Mer, Maillane, Saint-Julien-de-Peyrolas, Balaruc-les-Bains, Rennes-les-Bains, Arles-sur-Tech, Vaison, Die, La Batie-Montsaleon, La Buissse, Uriage, Albens, Servoz, Jussy, Vieu, Briord, Lectoure, Saint-Paul-les-Dax, Saintes, Fouras, Fouqueure, Mont de Jouer, Evaux, Saint-Sulpice-le-Gueretois, MOntoncelle, Herbet, Le Mont-Dore, Saint-Paulien, Sainte-Solange, La Grange-saint-Jean, Saint-Just-sur-Auron, Drevant, Beaulieu, Thesee, Suevres, Luynes, Contre, Blere, Courcay, Chiesseaux, Brizay..... and many more.
With all these aqueducts - by one count, well over a hundred throughout Gaul - it would have been hard for anyone who did get around the region not to see at least a few.
Survival-wise, aqueducts had a few strikes against them. Since towns typically taxed their use, people were already doing what many do with the electric grid today: tapping in illegally. Then, as order began to disintegrate (well before the "barbarians" took over), some simply fell into disrepair (they required regular upkeep). They were also tempting targets for scavengers. Supposedly Lugdunum (now Lyon) had to leave its original site on the top of a hill after either thieves or barbarian invaders took all the (valuable) lead used for pipes - recalling the cases where whole banks of public lights have gone dark after thieves stole the copper wiring.
The theft of lead piping had other implications, these for historians. Valuable information has been gleaned from those in Rome itself:
Prof. Rodolfo Lanciani, of Rome, has made a special study of these Roman lead pipes, and by means of the inscriptions, in raised letters, ordinarily found upon them, and evidently produced by engraved rollers used in rolling the lead plates out of which the pipes were made, has made some curious discoveries. He has located by means of them the residences of 80 or 90 distinguished citizens. He also finds that there were female plumbers in ancient Rome as well as female householders; but whether a female plumber in ancient times was any more reliable than the male plumber of modern times, the records so far discovered do not say.
The Cornell Civil Engineer, Volumes 1-4 "Frontinus, and his II. Books on the Water-Supply of the City of Rome 15-54There is also the question of how necessary aqueducts - which often fed the Roman baths - were to a city's survival. A nineteenth century visitor to the original site of Lyons claimed that it contained "several springs of excellent water", making the damaged aqueducts potentially less essential than one might think.
Nor were all aqueducts built by the Romans. The one at Ansignan may have been built before their rule, by the Volks Tectosages. Nor were all above ground; some were underwater conduits.
Aqueducs also played an important role in sieges. Supposedly the bridge mill was invented when Rome was besieged in the sixth century and the Goths shut off the aqueducts - prompting the Byzantine commander within the city, Belisarius, to invent the bridge mill to use the water of the river to grind grain. Gregory de Tours tells how a Burgundian king, besieged in Vienne, unwisely expelled all the poor - including the man in charge of the aqueduct, who proceeded to tell his attacker how to enter the city through that conduit.
How long into the Middle Ages did these wonders of engineering last? Many no doubt decayed early on; for others we have no clear data. But by one account, some survived in Vienne until 883; that at Luynes was still being repaired in the same century. In the nineteenth century, efforts were made to restore that at Besancon; several others were still in sufficiently usable state for similar initiatives to be attempted.
It seems then that at least some of the aqueducts still would have functioned, and even for some time, under the Franks. Even those which had fallen into disuse would have remained prominent in the landscape, one more reminder of how Roman early medieval France was.
Adrien Blanchet, Recherches sur les aqueducs et cloaques de la Gaule romaine
Belgrand, Eugène, 1810-1878, Les eaux. Introduction. Les aqueducs romains (1875)
C. Germain de Montauzan, Les aqueducs antiques de Lyon : étude comparée d'archéologie romaine : thèse présentée à la Faculté des lettres de l'Université de Paris
Herschel, "Frontinus, and his II books", The Cornell Civil Engineer, Volumes 1-4 II:15
Burton, "Antiquities of Rome", Galignani's magazine and Paris monthly review, Volume 5, 1823, 247
Thomas Mermet, Histoire de la ville de Vienne: Dauphiné, Volume 2
Gregorius Turonensis, Saint Gregory (Bishop of Tours), Joseph Guadet, Nicolas-Rodolphe Taranne, Histoire ecclésiastique des Francs, Volume 1