The Rio Ouro Preto Biological Reserve is in the municipality of Guajará-Mirim, Rondônia.

[4] It is in the pediplain of the centre of western Brazil, with altitudes from 100 to 300 metres (330 to 980 ft) above sea level, in the Amazon biome.

It includes open submontane rainforest (54.82%), dense montane Savana of the Pacaás Novos (16.58%), rainforest-savanna transition (14.52%) and arboreal savanna (5.21%).

[4] The Rio Ouro Preto Biological Reserve was created by decree 4.580 of 28 March 1990 with an area of about 46,438 hectares (114,750 acres), administered by the Rondônia Secretariat of State of the Environment (SEMARO).

The act included a caveat that the armed forces and federal police had freedom of transit and access to undertake their duties of national security and public law enforcement, including installation of infrastructure such as access roads.