The Chay River, the second largest tributary of the Lo River, which in turn joins the Mekong River, rises from the Tay Con Linh mountain range (in Vietnam) at a height of . After flowing towards the west, it flows further southeast in the middle and lower reaches, at which point it joins the Lô River at Đoan Hùng. The basin is delimited by high mountain ranges—in the north by ranges of to in height, on the west by the Con Voi mountain range and on the east and south east by the Tay Con Linh range. The width of the river is wide in its upper reaches and "tapers to an elongated shape in the middle and lower reaches". The basin has an area of 6500 km (1620 km in China) with 60% area below an elevation of 400 m (average elevation of the basin is 858 m). The annual rainfall in the basin varies from 1,500 mm to 3,000 mm with rains concentrated (75–85%) during May to October.