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The Bolshoy Uzen (Kazakh: Үлкен Өзен Úlken Ózen, Russian: Большой Узень) is a river in Saratov Oblast of Russia and West Kazakhstan Province of Kazakhstan. It is 650 kilometres (400 mi) long, with a drainage basin of 15,600 square kilometres (6,000 sq mi).
The Bolshoy Uzen has its sources on the western edge of the Obshchy Syrt highlands of Russia, and flows in a generally southerly direction over the steppes of the Caspian Depression. It terminates in West-Kazakhstan, in a network of small lakes and swamps called the Kamysh-Samarskiye Lakes. The Bolshoy Uzen runs parallel to the Maly Uzen River, some 50 kilometres (31 mi) further west.
Most of the river's waters comes from snowmelt, so the river's flow peaks in April, while in the summer parts of the river will dry up completely. At Novouzensk the discharge varies from 7.3 to 393 cubic metres per second (260 to 13,880 cu ft/s). The river freezes over in December and stays icebound to the end of March or the beginning of April.
The town of Novouzensk is situated at the Bolshoy Uzen. The river is used for water supply and irrigation.