Having hurried ahead of the main body of the expedition, Meriwether Lewis and four men arrive at the Great Falls of the Missouri River, confirming that the explorers are headed in the right direction.

Meriwether Lewis and William Clark had set out on their expedition to the Pacific the previous year. They spent the winter of 1804 with the Mandan Indians in present-day North Dakota. The Hidatsa Indians, who lived nearby, had traveled far to the West, and they proved an important source of information for Lewis and Clark. The Hidatsa told Lewis and Clark they would come to a large impassable waterfall in the Missouri when they neared the Rocky Mountains, but they assured the captains that portage around the falls was less than half a mile.

Armed with this valuable information, Lewis and Clark resumed their journey up the Missouri accompanied by a party of 33 in April. The expedition made good time, and by early June, the explorers were nearing the Rocky Mountains. On June 3, however, they came to a fork at which two equally large rivers converged. “Which of these rivers was the Missouri?” Lewis asked in his journal. Since the river coming in from the north most resembled the Missouri in its muddy turbulence, most of the men believed it must be the Missouri. Lewis, however, reasoned that the water from the Missouri would have traveled only a short distance from the mountains and, therefore, would be clear and fast-running like the south fork.

The decision was critical. If the explorers chose the wrong river, they would not be able to find the Shoshone Indians from whom they planned to obtain horses for the portage over the Rockies. Although all of their men disagreed, Lewis and Clark concluded they should proceed up the south fork. To err on the side of caution, however, the captains decided that Lewis and a party of four would speed ahead on foot. If Lewis did not soon encounter the big waterfall the Hidatsa had told them of, the party would return and the expedition would backtrack to the other river.

On this day in 1805, four days after forging ahead of the main body of the expedition, Lewis was overjoyed to hear “the agreeable sound of a fall of water.” Soon after he “saw the spray arise above the plain like a column of smoke…. [It] began to make a roaring too tremendous to be mistaken for any cause short of the great falls of the Missouri.” By noon, Lewis had reached the falls, where he stared in awe at “a sublimely grand specticle [sic]… the grandest sight I had ever held.”

Lewis and Clark had been correct—the south fork was the Missouri River. The mysterious northern fork was actually the Marias River. Had the explorers followed the Marias, they would have traveled up into the northern Rockies where a convenient pass led across the mountains into the Columbia River drainage. However, Lewis and Clark would not have found the Shoshone Indians nor obtained the horses. Without horses, the crossing might well have failed.

Three days after finding the falls, Lewis rejoined Clark and told him the good news. However, the captains’ elation did not last long. They soon discovered that the portage around the Great Falls was not the easy half-mile jaunt reported by the Hidatsa, but rather a punishing 18-mile trek over rough terrain covered with spiky cactus. The Great Portage, as it was later called, would take the men nearly a month to complete. By mid-July, however, the expedition was again moving ahead. A month later, Lewis and Clark found the Shoshone Indians, who handed over the horses that were so critical to the subsequent success of their mission.