Aach (toponymy)
Aach is a widespread Upper German hydronym, from an Old High German aha "running water".
The word has also been reduced to a frequent sufix -ach in Alemannic and Austro-Bavarian toponymy.
The word is cognate with Old English :wikt:æ#Old_English|ǣ, Old Frisian ē, Old Saxon aha, Low Franconian Aa, Old Norse á, Gothic aƕa, all meaning "river; running water".
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 "river island, wetland, floodplain, riparian woodland", 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, 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 in aar, ahr, acher reflect a cognate Celtic word.
