AA, 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.
Source: An unnamed encyclopedia from a project that puts out-of-copyright texts into the public domain.
This is from a *very* old source, and reflects the thinking of the turn of the last century.