Its chief importance is defining part of the boundary between Eritrea and Ethiopia, between the point where the Mai Ambassa enters the river at 14°53.6′N 37°54.8′E / 14.8933°N 37.9133°E / 14.8933; 37.9133 to the confluence of the Balasa with the Mareb at 14°38′N 39°1.3′E / 14.633°N 39.0217°E / 14.633; 39.0217.

[2] Other sources talking about a catchment of 21,000 square kilometres (8,100 sq mi) to 44,000 square kilometres (17,000 sq mi) over all, and a discharge of 21.6 cubic metres per second (760 cu ft/s) in average over the year, and 870 cubic metres per second (31,000 cu ft/s) in peaks.

It flows south, bordering Ethiopia, then west through western Eritrea to reach the Sudanese plains near Kassala.

Unlike the Setit or Takazze rivers, which flow out of Ethiopia and also forms a natural border with Eritrea, the waters of the Mareb do not usually reach the Nile[4] but dissipate in the sands of the eastern Sudanese plains, forming an inland delta.

The Mareb was important historically as the boundary between two separately governed regions in the area: the land of the Bahr negash (Tigrinya "kingdom of the sea", also known as Medri Bahri or "land of/by the sea") to the north of the river, and the Tigray to the south.