The Rio Branco is the principal affluent

Tributary

A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...

 of the Rio Negro (engl. Black river) from the north; it is enriched by many streams from the sierras which separate Venezuela

Venezuela

Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...

 and Guyana

Guyana

Guyana , officially the Co-operative Republic of Guyana, previously the colony of British Guiana, is a sovereign state on the northern coast of South America that is culturally part of the Anglophone Caribbean. Guyana was a former colony of the Dutch and of the British...

 from Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

. Its two upper main tributaries are the Urariquira and the Takutu

Takutu River

The Takutu River is a river in the Upper Takutu-Upper Essequibo Region of Guyana. It forms part of the boundary with Brazil, and is a tributary of the Branco River...

. The latter almost links its sources with those of the Essequibo

Essequibo River

The Esequibo River is the largest river in Guyana, and the largest river between the Orinoco and Amazon. Rising in the Acarai Mountains near the Brazil-Guyana border, the Essequibo flows to the north for 1,010 km through forest and savanna into the Atlantic Ocean.-Geography:There are countless...

.

The Branco flows nearly south, and finds its way into the Negro through several channels and a chain of lagoons similar to those of the latter river. It is 350 miles (563.3 km) long, up to its Urariquira confluence. It has numerous islands, and, 235 miles (378.2 km) above its mouth, it is broken by a bad series of rapids.