The Rio Roosevelt, also historically known as the Rio da Dúvida (River of Doubt), is a significant tributary of the Aripuanã River in Mato Grosso, Brazil, with a length of approximately 760 kilometers. Its primary historical and cultural significance stems from the 1914 Roosevelt-Rondon Scientific Expedition, which marked the first exploration of the river by non-natives, led in part by Theodore Roosevelt. This legacy defines it as a remote and challenging expeditionary waterway. Access to the river, particularly its upper sections, requires specific permission from the indigenous Cinta Larga people, a critical regulation for any paddling trip. The river's character is defined by whitewater, with rapids ranging up to Class V, demanding advanced technical skill. Expeditions often utilize multi-day, self-supported formats, employing gear such as oar frame rafts and kayaks for journeys lasting around 12 days or more. The environment is that of a dense Amazonian tributary, with notable wildlife including large catfish. The river's aesthetic is one of a narrow, winding channel through pristine rainforest, with few to no developed facilities or amenities for paddlers, emphasizing a complete wilderness experience. The best season for paddling is not explicitly detailed, but planning must account for the significant logistical and permitting hurdles associated with accessing this remote and culturally sensitive region.