Draining an area of just under 1million km2 or 77% of the land area of South Africa, the Orange River has its source in the Drakensberg mountains, but starts as the Senqu River in Lesotho. From here it flows westwards to the Atlantic Ocean. The headwaters are located at an altitude of 3300 m and consequently parts freeze in the winter months. After the town of Kimberly the river is joined by the Vaal river, its main tributary and from here it enters the arid region of the southern Kalahari and Namib desert. Dams along the river provide water for irrigation and hydropower, however because of the unpredictable flow and sand bar at the river mouth navigation is limited.