The Course

Orange
Gariep, Oranje, Senqu
River
Sunset over the Orange River near Upington in the Northern Cape
Countries	Lesotho, South Africa, Namibia
Tributaries
- right	Caledon River, Vaal River,Fish River (Namibia)
Landmarks	Gariep Dam, Augrabies Falls
Source	Thaba Putsoa [1]
- location	Maloti Mountains(Drakensberg), Lesotho
- elevation	3,350 m (10,991 ft)
Mouth	Alexander Bay
- location	Atlantic Ocean
Length	2,200 km (1,367 mi)
Basin	973,000 km2 (375,677 sq mi)
The course and watershed of the Orange River, Caledon River and Vaal River. This map shows a conservative border for the watershed. Specifically, the Kalahari basin is excluded, as some sources say it isendorheic.[2] Some other sources using computational methods show a basin which includes parts of Botswana (and hence of the Kalahari).[3]

The Orange River (Afrikaans/Dutch:Oranjerivier), Gariep River, Groote River orSenqu River is the longest river in South Africa. It rises in the Drakensberg mountains inLesotho, flowing westwards through South Africa to the Atlantic Ocean. The river forms part of the international borders between South Africa andNamibia and between South Africa and Lesotho, as well as several provincial borders within South Africa. Except for Upington, it does not pass through any major cities. The Orange River plays an important role in the South African economy by providing water for irrigation, as well as hydroelectric power. The river was named by Robert Jacob Gordon after the Dutch Royal House.

Course[edit]

The Orange rises in the Drakensberg mountains along the border between South Africa andLesotho, about 193 km (120 mi) west of the Indian Ocean and at an altitude of over 3,000 m. While in Lesotho, the river is known as the Senqu and parts of it freeze in winter, because of the high altitude there. This creates droughts downstream of it which mainly affects goat and cattle production.

The river then runs westward through South Africa, forming the south-western boundary of the Free State province. In this section the river flows first into the Gariep Dam (the largest in the country), and later into the Vanderkloof Dam. From the border of Lesotho to below the Van der Kloof Dam the river bed is deeply incised. Further downstream the land is flatter, and the river is used extensively for irrigation.

At the western point of the Free State, southwest of Kimberley, the Orange meets with its main tributary, the Vaal River, which itself forms much of the northern border of the province. From here the river flows further westward through the arid wilderness of the southernKalahari region and Namaqualand in the Northern Cape Province to meet with Namibia at the 20th degree of east longitude. From here it flows westward for 550 km,^[4] forming theinternational border between the province and Namibia's ǁKaras Region. On the border, the river passes the town of Vioolsdrif, the main border post between South Africa and Namibia.

In the last 800 km (500 mi) of its course the Orange receives many intermittent streams and several large wadis lead into it. In this section, the Namib Desert terminates on the north bank of the river, so under normal circumstances the volume of water added by these tributaries is negligible. Here the bed of the river is once again deeply incised. The Hundred Falls orAugrabies Falls are located on this section of the Orange, where the river descends 122 m (400 ft) in a course of 26 km (16 mi).

This image shows only the last c. 100 kilometers of the Orange River. In this last stretch the gravel deposits in the river bed and along the banks are rich with diamonds, and several diamond mines operate along the stretch pictured here.

The Orange empties into the Atlantic Ocean at Alexander Bay, which lies about equidistant between Cape Townand Walvis Bay. Some 33 km (21 mi) from its mouth it is completely obstructed by rapids and sand bars and is generally not navigable for long stretches.

The river has a total length of 2,200 km (1,400 mi).

Catchment and rainfall[edit]

The Augrabies Falls in full flow.

In the dry winter months the volume of the water in the river is considerably reduced because of the rapid run-off andevaporation. At the source of the Orange the rainfall is approximately 2,000 mm per annum but precipitation decreases as the river flows westward; at its mouth the rainfall is less than 50 mm per annum. The factors that support evaporation, on the other hand, tend to increase in a westerly direction. However, in the wet season (summer) the Orange river becomes an impetuous, brown torrent. The huge mass of sediment carried constitutes a long-term threat to all engineering projects on the river.^[5]

The total catchment of the Orange River (including the Vaal) extends over 973,000 km², i.e. equivalent to about 77% of the land area of South Africa (1,268,5358 km²). Approximately 366,000 km² (38%), however, is situated outside the country in Lesotho, Botswana and Namibia.