The Aare () or Aar () is a tributary of the High Rhine and the longest river that both rises and ends entirely within Switzerland. Its total length from its source to its junction with the Rhine comprises about , during which distance it descends , draining an area of , almost entirely within Switzerland, and accounting for close to half the area of the country, including all of Central Switzerland. There are more than 40 hydroelectric plants along the course of the Aare. The river's name dates to at least the La Tène period, and it is attested as Nantaror "Aare valley" in the Berne zinc tablet. The name was Latinized as Arula/Arola/Araris. Course The Aare rises in the great Aargletschers (Aare Glaciers) of the Bernese Alps, in the canton of Bern and west of the Grimsel Pass. The Finsteraargletscher and Lauteraargletscher come together to form the Unteraargletscher (Lower Aar Glacier), which is the main source of water for the Grimselsee (Lake of Grimsel). The Oberaargletscher (Upper Aar Glacier) feeds the Oberaarsee, which also flows into the Grimselsee. The Aare leaves the Grimselsee just to the east to the Grimsel Hospiz, below the Grimsel Pass, and then flows northwest through the Haslital, forming on the way the magnificent Handegg Waterfall, , past Guttannen. Right after Innertkirchen it is joined by its first major tributary, the Gamderwasser. Less than later the river carves through a limestone ridge in the Aare Gorge (). It is here that the Aare proves itself to be more than just a river, as it attracts thousands of tourists annually to the causeways through the gorge. A little past Meiringen, near Brienz, the river expands into Lake Brienz. Near the west end of the lake it indirectly receives its first important tributary, the Lütschine, by the Lake of Brienz. It then runs across the swampy plain of the Bödeli (Swiss German diminutive for ground) between Interlaken and Unterseen before flowing into Lake Thun. Near the west end of Lake Thun, the river indirectly receives the waters of the Kander, which has just been joined by the Simme, by the Lake of Thun. Lake Thun marks the head of navigation. On flowing out of the lake it passes through Thun, and then flows through the city of Bern, passing beneath eighteen bridges and around the steeply-flanked peninsula on which the Old City is located. To the south