Aach (variants Ach, Ache; Aa) is a widespread Upper German hydronym, from an Old High German German, Old High (ca.750-1050);: aha (Proto-Germanic Germanic languages: [[:wikt:Appendix:Proto-Germanic/ahwō|ahwō]]) (ultimately from PIE Indo-European languages: [[:wikt:Appendix:Proto-Indo-European/h₂ekʷeh₂|hakʷā-]]).The word has also been reduced to a frequent suffix -ach in Alemannic and Austro-Bavarian toponymy.The word is cognate with Old English English, Old (ca.450-1100);: [[:wikt:æ#Old English|ǣ]] (reflected in English placenames as English: -ea, also Yeo, Eau), Old Frisian ē, Old Saxon aha, Low Franconian Aa, Old Norse Norse, Old: á, Gothic Gothic: aƕa, all meaning .
The Old High German contraction from German, Old High (ca.750-1050);: -aha to German, Old High (ca.750-1050);: -aa, German, Old High (ca.750-1050);: -â in compound hydronyms present from an early time (early 9th century). The simplex noun German, Old High (ca.750-1050);: aha remained uncontracted, however, and Old High German German, Old High (ca.750-1050);: -aha (Modern German German: -ach) could be restituted in compounds at any time.[1]
Related is the German German: Aue (variant German: Au) with a meaning, i.e. a cultivated landscape in a riparian zone. It is derived from the same root, but with a German, Old High (ca.750-1050);: -yo- suffix (Proto-Germanic Germanic languages: awjō). This word was also reduced to a suffix, as German: -au (as in Reichenau). It is frequent as a river name, as in Große 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 German: Aach in Upper Germany is reserved for broad, but non-navigable, running streams with noticeable gradient sufficient to power water mills; it contrasts with German: Fluss used for navigable rivers on one hand, and with German: Bach for minor brooks or rivulets. An instructive example is Salzach, now classed as a German: Fluss but formerly as German: Ache as it was only navigable by raft, not by regular riverboats.
Hydronymy in German: -ach generally indicates a Germanic settlement in the early medieval or migration period, while names in German: -bach indicate names of the high medieval period.[2] In French, the Old Frankish form evolved into French: aix, as in Aix-en-Provence, Aix-les-Bains; the Italian reflex is Italian: -acco.Hydronyms such as Aar, Ahr, and Acher reflect a cognate Celtic word.