On the evening of June 2, 1805, the Corps of Discovery arrived at the mouth of an unknown river flowing from the north. The river had not been mentioned by the Hidatsa and was not noted on their maps, and the two rivers were equal in size, raising the question of which was the true Missouri. The Captains decided to explore both forks, with Lewis going up the north branch on June 4 and Clark journeying to the south. After several days of exploration, they decided that the south fork was the true Missouri, and the north fork was named for Maria Wood, Lewis's cousin. They made this decision despite objections from the rest of the Corps, though the men agreed to follow them anywhere. A wrong decision may have doomed the expedition, but a few days later Captain Lewis found the Great Falls and the Captains were vindicated. The region lay in the heart of Blackfoot Country and was a favored location for winter camps of the Piikani. Fort Piegan was established in 1831, and the area became an important fur trade region.