Theiss (Hungarian, Tisza; Lat., Tisia or Tissus), a large affluent of the Danube, next to which it is the greatest river of Hungary. It rises in the north-eastern part of the Carpathian mountains, in the county of Maramaros, at a height of above 6300 ft., and is formed by the confluence of two branches, the Black Theiss (Fekete Tisza), and the White Theiss (Feher Tisza), which unite at about 20 m. E. of Maramaros-Sziget. The Theiss then follows a north-westerly direction until it leaves its mountainous valley, then runs west, and after a great curve to the north, takes a south-westerly direction and enters the great Hungarian plain (Alf old). From Szolnok it runs south in an almost parallel course with that of the Danube, from which it is separated by a distance of about 60 m., and flows into the Danube near the village of Titel, 20 m. E. of Ujvidek. Its length from source to mouth is, as the crow flies, only about 340 m., but its windings make its course about 870 m. long. The Theiss is clear and swift in its course through the mountains, but in the plain it becomes slow, somewhat muddy and very tortuous. Its basin covers an area of 56,600 sq. m., and comprises the whole eastern part of Hungary, and the greater part of Transylvania, and collects all the rivers descending from the Carpathians westward.
The Theiss is navigable for rafts almost everywhere, but for steamers only from Szolnok downwards, a distance of about zoo m., where the breadth of the river is 450 to 750 ft. The depth of the Theiss at low-water mark is 7 ft. at Tokaj, 20 ft. at Szeged and II ft. at Titel, near its mouth, while the difference between the low-water mark and the high-water mark is as high as 25 to 35 ft. During its course through the great Hungarian plain the Theiss flows between flat, low-lying banks, which are the cause of periodical and sometimes disastrous inundations and of extensive marshes. Therefore extensive works have been undertaken for the regulation and canalization of the river, which is now strongly dammed in many parts. By these works large tracts of marshes have been transformed into productive ground. Its chief tributaries are the Szamos, Koros, Maros, Latorcza, and the Sajo. In its lower course it is joined to the Danube by the Franz Josef canal, while it is also united with Temesvar by the Bega canal.
