Question: Where is the origin of the river that Orlam clans crossed to migrate to Namibia?

Knowledge 1: Namibia
The dry lands of Namibia were inhabited since early times by San, Damara, Nama and, since about the 14th century AD, by immigrating Bantu who came with the Bantu expansion from central Africa. From the late 18th century onwards, Orlam clans from the Cape Colony crossed the Orange River and moved into the area that today is southern Namibia. Their encounters with the nomadic Nama tribes were largely peaceful. The missionaries accompanying the Orlams were well received by them, the right to use waterholes and grazing was granted against an annual payment. On their way further northwards, however, the Orlams encountered clans of the Herero tribe at Windhoek, Gobabis, and Okahandja which were less accommodating. The Nama-Herero War broke out in 1880, with hostilities ebbing only when Imperial Germany deployed troops to the contested places and cemented the status quo between Nama, Orlams, and Herero. [SEP] Knowledge 2: Orange River
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 - 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 km (375,677 sq mi) Discharge - average 365 m / s (12,890 cu ft / s) 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 is endorheic. Some other sources using computational methods show a basin which includes parts of Botswana (and hence of the Kalahari).
Knowledge 1: Namibia
The dry lands of Namibia were inhabited since early times by San, Damara, Nama and, since about the 14th century AD, by immigrating Bantu who came with the Bantu expansion from central Africa. From the late 18th century onwards, Orlam clans from the Cape Colony crossed the Orange River and moved into the area that today is southern Namibia. Their encounters with the nomadic Nama tribes were largely peaceful. The missionaries accompanying the Orlams were well received by them, the right to use waterholes and grazing was granted against an annual payment. On their way further northwards, however, the Orlams encountered clans of the Herero tribe at Windhoek, Gobabis, and Okahandja which were less accommodating. The Nama-Herero War broke out in 1880, with hostilities ebbing only when Imperial Germany deployed troops to the contested places and cemented the status quo between Nama, Orlams, and Herero. [SEP] Knowledge 2: Omaruru River
The Omaruru River is a major river crossing the Erongo Region of western central Namibia from East to West. It originates in the Etjo mountains, crosses the city of Omaruru and reaches the sea a few kilometers north of Henties Bay.
Knowledge 1: Namibia
The dry lands of Namibia were inhabited since early times by San, Damara, Nama and, since about the 14th century AD, by immigrating Bantu who came with the Bantu expansion from central Africa. From the late 18th century onwards, Orlam clans from the Cape Colony crossed the Orange River and moved into the area that today is southern Namibia. Their encounters with the nomadic Nama tribes were largely peaceful. The missionaries accompanying the Orlams were well received by them, the right to use waterholes and grazing was granted against an annual payment. On their way further northwards, however, the Orlams encountered clans of the Herero tribe at Windhoek, Gobabis, and Okahandja which were less accommodating. The Nama-Herero War broke out in 1880, with hostilities ebbing only when Imperial Germany deployed troops to the contested places and cemented the status quo between Nama, Orlams, and Herero. [SEP] Knowledge 2: Namibia
Namibia (i/nəˈmɪbiə/, /næˈ-/), officially the Republic of Namibia (German:  Republik Namibia (help·info); Afrikaans: Republiek van Namibië) is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Zambia and Angola to the north, Botswana to the east and South Africa to the south and east. Although it does not border Zimbabwe, a part of less than 200 metres of the Zambezi River (essentially a small bulge in Botswana to achieve a Botswana/Zambia micro-border) separates it from that country. Namibia gained independence from South Africa on 21 March 1990, following the Namibian War of Independence. Its capital and largest city is Windhoek, and it is a member state of the United Nations (UN), the Southern African Development Community (SADC), the African Union (AU), and the Commonwealth of Nations.

Answer: Thaba Putsoa