The Onkobra River was a river in the Imperial province of Black Marsh, or Argonia as it was known to the native Argonians. It flowed from Topal Bay inland into the swampy heart of Black Marsh, and was said to possess a strong current. It was located near the Argonian city of Gideon.
During the Third Era, an Imperial road ran parallel to the river. The Onkobra was also bordered by the estates and plantations of the Archeins, native Argonians who governed the rural areas of Black Marsh, as well as farms, orchards, and vineyards.