a river in Nepal, India, and, in its upper course, China; a left tributary of the Ganges. Length, 730 km; basin area, 86,900 sq km. Its sources lie in the Greater Himalayas, and it flows across the Indo-Gangetic Plain. Its waters are swelled to flood stage in the summer by melting snow and glaciers and by mon-soon rains. The average annual water flow near the river’s mouth is 1,770 cu m per sec. There is a hydrotechnical complex in Bihar State (India) and Nepal. The Kosi is navigable along its lower course.