Formerly called Rio da Dúvida (“River of Doubt”), the river is named after Theodore Roosevelt, who travelled into the central region of Brazil during the Roosevelt-Rondon Scientific Expedition of 1913. Roosevelt, with Brazil's most famous explorer, Cândido Rondon, explored the "River of Doubt," which Rondon had discovered on a previous expedition. The goal was to determine if the river flowed into the Amazon River, which it did via the Aripuana River. Sections of the river have impassable rapids and waterfalls, which hindered the expedition.
Roosevelt, and his son Kermit Roosevelt, travelled down the river after Teddy's failed attempt running as the "Bull Moose" candidate in 1912 for the office of President of the United States of America. Roosevelt's party was the first non Amazonian-native party to travel and record the "Rio da Duvida." Along with Brazil's most famous explorer, Candido Mariano da Silva Rondon, Roosevelt explored one of the most intimidating tributaries of the Amazon at that time. Rondon (the man known for planting telegraph poles across his native South American country), along with Roosevelt, went on one of the most dangerous journeys that existed in their day. Despite unbelievable hardships they were able to plot one of the longest tributaries of the Amazon.
Teddy Roosevelt later wrote a book on the expedition titled Through the Brazilian Wilderness. The story is also recounted in The River of Doubt by Candice Millard.