Wikipedia: Aar

Its total length (including all bends) from its source to its junction with the Rhine is about 291 km (181 miles), during which distance it descends 1565 m (5135 feet), while its drainage area is 17,620 km² (6804 square miles).

It rises in the great Aar glaciers, in the canton of Bern?, and west of the Grimsel Pass. It runs east to the Grimsel Hospice, and then northwest through the Hasli valley, forming on the way the magnificent waterfall of the Handegg, 46 m (151 feet), past Guttannen, and pierces the limestone barrier of the Kirchet by a grand gorge, before reaching Meiringen?, situated in a plain. A little beyond, near Brienz?, the river expands into the lake of Brienz, where it becomes navigable. Near the west end of that lake it receives its first important affluent, the Lutschine (left), and then runs across the swampy plain of the Bodoli, between Interlaken? (left) and Unterseen? (right), before again expanding in order to form the Lake of Thun.

Near the west end of that lake it receives on the left the Kander, which has just before been joined by the Simme; on flowing out of the lake it passes Thun, and then circles the lofty bluff on which the town of Bern is built. It soon changes its northwesterly for a due westerly direction, but after receiving the Saane or Sarine (left) turns north till near Aarberg its stream is diverted west by the [Hagneck Canal]? into the Lake of Bienne, from the upper end of which it issues through the Nidau Canal and then runs east to Buren?.

Henceforth its course is northeast for a long distance, past Soleure? (below which the Grosse Emme flows in on the right), Aarburg (where it is joined by the Wigger, right), Olten, Aarau, near which is the junction with the Suhr on the right, and Wildegg, where the Hallwiler Aa falls in on the right. A short way beyond, below Brugg, it receives first the Reuss (right), and very shortly afterwards the Limmat or Linth (right). It now turns due north, and soon becomes itself an affluent of the Rhine (left), which it surpasses in volume when they unite at Coblenz?, opposite Waldshut?.