The Ilek River (Kazakh: Елек, Russian: Илек) is a steppe river at the southern end of the Ural Mountains. It is a tributary of the Ural River and lies in the Orenburg Oblast in Russia and Republic of Kazakhstan. It rises just south of Orsk, flows south a short distance and then flows westward south of and parallel to the Ural River, with many meanders and oxbow lakes, and joins the Ural about 75 kilometres (47 mi) west of Orenburg. Two main cities lie on the banks of the Ilek River: Sol-Iletsk and Aqtöbe (alternate spelling: Aktöbe, Aktyubinsk). The Ilek River remains the most polluted water body in the Ural-Caspian basin. The content of boron and chromium in the river is caused by the tailing ponds of former chemical plants via ground water. The quality class of water in the Ilek River changes from 4 – “polluted water” to 6 – “very polluted water” ("Water resources of Kazakhstan in the new millennium," Water Resources Committee of RK, 2002). A further tributary of the Ilek River is the Bol'saja Chobda. It is also of archeological significance since it has been discovered as the site of certain Kurgan (Indo-European) burials. .