From there, it flows eastward towards the Rio Negro, serving as the border between the Rio Unini Extractive Reserve Extractive Reserve to the north and the Jaú National Park to the south.

[1] The river basin has about 1,500 streams and over 1,000 lakes, with an estimated length of 400 kilometres (250 mi) from its mouth to the headwaters.

[2] The Unini River is characterized by its generally acidic water with minimal suspended material.

Its meandering course, along with its tributaries, gives rise to a diverse array of aquatic environments, including lakes, rivers, creeks, and flooded forests.

The range of water levels vary from minimum to maximum between 11.26 and 15.75 metres (36.9 and 51.7 ft).