<P> On the south side of the Columbia River, 2 miles (3 km) upstream on the west side of the Netul River (now Lewis and Clark River), they constructed Fort Clatsop . They did this not just for shelter and protection, but also to officially establish the American presence there, with the American flag flying over the fort . During the winter at Fort Clatsop, Lewis committed himself to writing . He filled many pages of his journals with valuable knowledge, mostly about botany, because of the abundant growth and forests that covered that part of the continent . The health of the men also became a problem, with many suffering from colds and influenza . </P> <P> Lewis was determined to remain at the fort until April 1, but was still anxious to move out at the earliest opportunity . By March 22, the stormy weather had subsided and the following morning, on March 23, 1806, the journey home began . The Corps began their journey homeward using canoes to ascend the Columbia River, and later by trekking over land . </P> <P> They made their way to Camp Chopunnish in Idaho, along the north bank of the Clearwater River, where the members of the expedition collected 65 horses in preparation to cross the Bitterroot Mountains, lying between modern - day Idaho and western Montana . However, the range was still covered in snow, which prevented the expedition from making the crossing . On April 11, while the Corps was waiting for the snow to diminish, Lewis' dog, Seaman, was stolen by Indians, but was retrieved shortly . Worried that other such acts might follow, Lewis warned the chief that any other wrongdoing or mischievous acts would result in instant death . </P> <P> On July 3, before crossing the Continental Divide, the Corps split into two teams so Lewis could explore the Marias River . Lewis' group of four met some men from the Blackfeet nation . During the night, the Blackfeet tried to steal their weapons . In the struggle, the soldiers killed two Blackfeet men . Lewis, George Drouillard, and the Field brothers fled over 100 miles (160 kilometres) in a day before they camped again . </P>

Who was first to discover the passage to travel across the plains through the rocky mountains