Not mentioned in the canon - but just about every town and city forms around a source of water, usually a river. So it's safe to assume some un-named river runs through the city and country of Quirm on its way to the Circle Sea, and to continue the French parody, why not call it the Inseine?
Damn, damn, damn....Edit
Terry has, in his latest novel Snuff, '''given Quirm its river that winds through the city and the nation. With a sort of logic, he has called it the Quire. Now hw the Hell do I rename this article to meet new canonical reality?
DatMaibe answers: 'Quirm's river'? Or keep it and say -In the main trouserleg of time it is named....while in the Pessimalian trouserleg...'
As the Seine winds through Paris, it makes a distinctly Gallic shape...