river in northeastern China. The Nen River is the principal tributary of the Sungari (Songhua) River (Sungari River), which is itself a tributary of the Amur River. The Nen rises in the area where the Da Hinggan (Da Hinggan Range) and Xiao Hinggan (Xiao Hinggan Range) ranges come together in northern Heilongjiang (Heilungkiang) province and the Inner Mongolia Autonomous Region. It then flows southward, forming part of the border between these two areas as it crosses the northern section of the Northeast (Manchurian) Plain (Manchuria) to join the Sungari just east of Da'an in Jilin (Kirin) province. It receives the waters of a great many tributaries that run off the eastern slopes of the Da Hinggan and the western slopes of the Xiao Hinggan ranges. Although frozen for some four months in winter and subject to serious flooding during the summer, especially around its confluence with the Sungari, the Nen is an important water route, navigable by small steamers as far as Qiqihar (Tsitsihar) and by small craft much farther to the north. The river was even more important in the early period of Chinese settlement in Heilongjiang in the 19th century, when rivers were virtually the only way to transport people and goods.

In the wide, flat Northeast Plain the gradient of the Nen is very low and the river's course meanders. The plain is subject to flooding during the spring thaw and again during the summer. The plain itself is wet and in places waterlogged, having many salt bogs, swamps, and brackish lakes. The total length of the Nen River is 725 miles (1,170 km).