Aach (variants Ach, Ache; Aa) is a widespread Upper German hydronym, from an Old High German (Proto-Germanic ) (ultimately from PIE ). The word has also been reduced to a frequent suffix in Alemannic and Austro-Bavarian toponymy. The word is cognate with Old English (reflected in English placenames as , also Yeo, Eau), Old Frisian , Old Saxon , Low Franconian Aa, Old Norse , Gothic , all meaning .
The Old High German contraction from to , in compound hydronyms present from an early time (early 9th century). The simplex noun remained uncontracted, however, and Old High German (Modern German ) could be restituted in compounds at any time.
Related is the German (variant ) with a meaning , i.e. a cultivated landscape in a riparian zone. It is derived from the same root, but with a suffix (Proto-Germanic ). This word was also reduced to a suffix, as (as in Reichenau). It is frequent as a river name, as in GroÃÂe Aue, Aue (Elbe), Aue (Weser), 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 in Upper Germany is reserved for broad, but non-navigable, running streams with noticeable gradient sufficient to power water mills; it contrasts with used for navigable rivers on one hand, and with for minor brooks or rivulets. An instructive example is Salzach, now classed as a () but formerly as as it was only navigable by raft, not by regular riverboats.
Hydronymy in generally indicates a Germanic settlement in the early medieval or migration period, while names in indicate names of the high medieval period. In French, the Old Frankish form evolved into , as in Aix-en-Provence, Aix-les-Bains; the Italian reflex is . Hydronyms such as Aar, Ahr, and Acher reflect a cognate Celtic word.