Lewis was thrilled to see the enormous waterfall, the Great Falls of the Missouri. It was 900 feet wide and 80 feet high with a "beautiful rainbow" just above the spray. Lewis called it "the grandest sight" he "ever beheld." More importantly, it meant that he and Clark had chosen the right fork in the Missouri River. All winter long they had been talking with Native Americans about a water route across the west. The Native Americans had described the territory in detail, including important landmarks like the waterfall in front of Lewis.

Now it was time for the portage (the carrying of goods over an obstacle): Lewis and Clark's expedition party needed to carry their canoes and supplies around the waterfalls. Before they saw the Great Falls, Lewis and Clark thought the portage would be less than a mile. As Lewis scouted further, he found four more waterfalls and realized that the portage would be much longer.