The Koss River is a river that flows in a north of northwest direction through the Eastern Equatoria state of South Sudan

South Sudan

South Sudan , officially the Republic of South Sudan, is a landlocked country located in the Sahel region of northeastern Africa. It is also part of the North Africa UN sub-region. Its current capital is Juba, which is also its largest city; the capital city is planned to be moved to the more...

, fed by streams from the Imatong Mountains

Imatong Mountains

The Imatong Mountains are located in the southeast of South Sudan in the state of Eastern Equatoria, and extend into Uganda. Mount Kinyeti is the highest mountain of the range at , and the highest in the whole of South Sudan....

 to the west.

The Koss river rises near Ikotos

Ikotos

Ikotos is a town in the Eastern Equatoria state of South Sudan, headquarters of Ikotos County.The town is home to the Lango people, who have a total population of 25,000 - 30,000 people in Ikotos county and elsewhere, and speak dialects of the Lotuko language....

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The head-waters of the river form a waterfall near Laborokala. This is the location of an annual Lango ceremony before the start of the rainy season where a goat is sacrificed and its stomach contents and later its bones are thrown into the stream.
The Shilok river, a tributary of the Koss that separates the southeastern part of the Imatong mountains from the Teretenya ridge, joins the Koss not far from its source.

The Koss river divides the lowland area east of the Imatong Mountains from that west of the Lafit, Dongotona and Nangeya Mountains. Streams from these mountains feed the river.
The river flows through the Bari

Bari people

The Bari ethnic groups in South Sudan occupy the Savanna lands of the White Nile Valley. They speak a language which is also called Bari. The name "Bari of the Nile Valley" would be fitting because the river Nile runs through the heart of the Bari land...

 country to the east of Torit

Torit

-Location:The town is located in Torit County, Eastern Equatoria State, in the southeastern part of South Sudan, close to the International border with the Republic of Uganda. Its location lies approximately , by road, east of Juba, the capital and largest city in that country...

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Near to Tirangore the river is spanned by a bailey bridge on the Torit-Kapoeta

Kapoeta

Kapoeta is a town in South Sudan. It is located in Kapoeta South County, in Eastern Equatoria State, in southeastern South Sudan.-Location:The town lies on the east bank of the Singaita River...

 road, built by the Norwegian Church Aid.
In 1881, Emin Pasha

Emin Pasha

Mehmed Emin Pasha — he was born Isaak Eduard Schnitzer and baptized Eduard Carl Oscar Theodor Schnitzer — was a physician, naturalist, and governor of the Egyptian province of Equatoria on the upper Nile...

 made a trip from Gondokoro

Gondokoro

Gondokoro was a trading-station on the east bank of the White Nile in Southern Sudan, 750 miles south of Khartoum. Its importance lay in the fact that it was within a few miles of the limit of navigability of the Nile from Khartoum upstream...

 on the Nile

Nile

The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...

 to "Tarangole", and from there travelled south along the Koss Valley, and then southwest to the Nile.
North of Tirangore, the river passes Lafon

Lafon, South Sudan

Lafon is a community in Eastern Equatoria State of South Sudan, the headquarters of Lafon County.The people belong to the Pari ethnic group.Lafon Hill is a small, rocky elevation that rises abruptly from the surrounding plain....

and then flows almost due north before being lost in marshland.