Its total length from its source to its junction with the Rhine comprises about 295 km (183 miles), during which distance it descends 1,565 m (5,135 ft), draining an area of 17,779 km² (6,865 mi²).
Near the west end of Lake Thun, the river receives the waters of the Kander, which has just been joined by the Simme. On flowing out of the lake it passes through Thun, and then flows through the city of Berne, passing beneath eighteen bridges and around the steeply-flanked peninsula on which the Old City is located. The river soon changes its northwesterly flow for a due westerly direction, but after receiving the Saane or Sarine it turns north until it nears Aarberg. There, in one of the major Swiss engineering feats of the 19th century the river, which had previously rendered the countryside north of Bern a swampland through frequent flooding, was diverted by the Hagneck Canal into Lake Biel. From the upper end of the lake the river issues through the Nidau Canal and then runs east to Büren. The lake absorbs huge amounts of eroded gravel and snowmelt that the river brings from the Alps, and the former swamps have become fruitful plains: they are known as the "vegetable garden of Switzerland".
From here the Aar flows northeast for a long distance, past the ambassador town Solothurn (below which the Grosse Emme flows in on the right), Aarburg (where it is joined by the Wigger), Olten, Aarau, near which is the junction with the Suhr, and Wildegg, where the Hallwiler Aa falls in on the right. A short distance further, below Brugg it receives first the Reuss, and shortly afterwards the Limmat. It now turns due north, and soon becomes itself an affluent of the Rhine, which it surpasses in volume when the two rivers unite at Koblenz (Switzerland), opposite Waldshut, Germany.
The Aar Gorge (Aareschlucht) is a section of the river that carves through a limestone ridge near the town of Meiringen. The gorge is an indirect product of glaciation; 10,000 years ago, just as the Ice Age was coming to an end, torrential runoff water from melting glaciers eroded a deep, narrow chasm through the limestone barrier. Although barely one mile long, this passage is bordered by sheer cliffs up to 165 feet (50 m) high on either side. At the bottom of this steep drop the river is scarcely three feet (one meter) wide.
The gorge has been opened to the public since 1889, by building walking paths along the Gorge. Before then, the only way was to go through the dangerous river torrent, which provided fables by travellers, who claimed to see large snakes and monsters. Since then, the gorge is a popular tourist attraction for many visitors who like natural beauty.