The Hyères sometimes causes serious floods: in March 1903, the Sainte-Catherine chapel located in Plounevézel but at the limit of Carhaix, had water up to the roof[5] and in 1910 to the stained glass windows.

In a charter signed by the Count of Cornouaille, Hoel, to "donate a villa near Caer Ahes, in which is the church of sanctus Kigavus (saint Quijeau),[9] we find the oldest form of name of Carhaix, close and contemporary to those mentioned in medieval novels.

The Breton name is Karaez (spelled Carahes in the eleventh century in a charter of Count Hoel, based on the prefix "Kaer" which means "fortified place").

It may be that Corophesium represents neither one nor the other, but corresponds, as Léon Fleuriot indicates in his book The Origins of Brittany (1987), to an error of the scribe.

Bernard Tanguy brings Karaes closer to Carofes, attested in Low-Latin for the name of the city of Diablintes and for Charroux (Vienna).

It would then be an old * Carofum / * Carofensis (evolution of quadruvium in carruvium), registering Carhaix in its function of road junction.

[11] Moreover, could the association of Charlemagne with Carhaix in the novel of Aiquin be based on the expedition of his son, Louis the Pious, about which is named Corophesium?

The permanence of the Carhaix crossroads function, together with its decline in the Lower Empire, may explain that, although the city was the capital of Osism, they did not leave their name as it was the case.

[12] In continental histories Carhaix is thought to be Carohaise of King Leodegrance and the Roman city of Vorgium.

Modern archaeological digs have uncovered evidence of the ancient Roman city including its aqueduct system.