The Unstrut is a 192 km long, left tributary of the Saale and its most water-rich tributary. Its catchment area is almost the entire Thuringian basin along with part of the western and northern peripheral plateaus, parts of the southern Harz and smaller parts of the northern Thuringian Forest. The more water-rich tributaries, like the Unstrut itself, have their source in the peripheral mountains, which have more precipitation than the basin.