Aa, the name of many small European rivers. The word is derived from the Old High German aha, cognate to the Latin aqua (“water”).
Among the streams of this nature are: a river in northern France flowing through St. Orner and Gravelines and a river of Switzerland, in the cantons of Lucerne and Aargau, which carries the waters of the "finger" lakes of Baldeggersee and Hallwilersee into the Aare. In Germany there are the Westphalian Aa, joining the Werre at Herford, the Münster Aa, a tributary of the Ems, and others. Two rivers in Latvia, the Lielupe and the Gauja, were formerly known as Curlandish and Livonian Aa, respectively.