Aach (toponymy)
Aach is a widespread Upper German hydronym, from an Old High German aha .
The word has also been reduced to a frequent suffix -ach in Alemannic and Austro-Bavarian toponymy.
The word is cognate with Old English ǣ, Old Frisian ē, Old Saxon aha, Low Franconian Aa, Old Norse á, Gothic aƕa, all meaning.
The Old High German contraction from -aha to -aa, -â in compound hydronyms present from an early time. The simplex noun aha remained uncontracted, however, and Old High German -aha could be restituted in compounds at any time.
Related is the German Aue with a meaning, i.e. a cultivated landscape in a riparian zone. It is derived from the same root, but with a -yo- suffix. This word was also reduced to a suffix, as -au. It is frequent as a river name, as in Große Aue, Aue, Aue, etc., as well as the name of a settlement, as in Aue, Saxony; Au, St. Gallen; Au, Vorarlberg; Au am Rhein; Au am Leithaberge; etc.
The river-name Aach in Upper Germany is reserved for broad, but non-navigable, running streams with noticeable gradient sufficient to power water mills; it contrasts with Fluss used for navigable rivers on one hand, and with Bach for minor brooks or rivulets. An instructive example is Salzach, now classed as a Fluss but formerly as Ache as it was only navigable by raft, not by regular riverboats.
Hydronymy in -ach generally indicates a Germanic settlement in the early medieval or migration period, while names in -bach indicate
names of the high medieval period.
In French, the Old Frankish form evolved into aix, as in Aix-en-Provence, Aix-les-Bains; the Italian reflex is -acco.
Hydronyms such as Aar, Ahr, and Acher reflect a cognate Celtic word.