Most of the rivers originate from the glaciers and are consequently heavily laden with debris, which make them turbid and often yellowish-brown in colour. The longest river, j rs in the south, is 230 km long and has an average discharge of 390 cubic metres per second. The second longest, J kuls Fj llum in the northeast, is 206 km long. Br in the east.
Icelandic rivers are chiefly of two types, glacial and clear-water rivers.
The former usually divide into numerous more or less intertwined tributaries that constantly change their course and swing over the outwashed plains lying below the glaciers. This is especially true of the rivers running south from Vatnaj kull. In this area it was extremely difficult to build a permanent road, since the bridges and parts of the roads were constantly being washed away when the glacial rivers reached their maximum discharge, usually in July and August. Clear-water rivers are of two kinds. One drains the old basalt areas and has a variable discharge with maximum flow in late spring.