1911 Encyclopedia Britannica
The name of a large number of small European rivers. The word is derived from the Old German aha, cognate to the Latin aqua, water (cf. Ger. -ach; Scand. a, aa, pronounced o). The following are the more important streams of this name: Two rivers in the west of Russia, both falling into the Gulf of Riga, near Riga, which is situated between them; a river in the north of France, falling into the sea below Gravelines, and navigable as far as St Omer; and a river of Switzerland, in the cantons of Lucerne and Aargau, which carries the waters of Lakes Baldegger and Hallwiler into the Aar. In Germany there are the Westphalian Aa, rising in the Teutoburger Wald, and joining the Werre at Herford, the Munster Aa, a tributary of the Ems, and others.
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Chisholm, Hugh, General Editor. Entry for 'Aa'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/a/aa.html. 1910.