Lewis and Clark Trail

By editor

Fort Benton, Chouteau County, Montana

The Missouri River below Fort Benton unrolled before the eyes of the Corps of Discovery not merely as a body of water but as a living highway carved by relentless forces over countless millennia. This river, coursing through the sedimentary plains of northern Montana, was their first trail west, guiding the expedition into regions sculpted by ancient seas, glaciers, and tectonic upheavals. It was upon these waters and their adjoining valleys that Meriwether Lewis and William Clark embarked in 1805, setting their sights on the Rocky Mountains’ forbidding barrier.

The Missouri here flows through strata of the Cretaceous period, where layers of shale and sandstone tell stories of an inland sea that once submerged this land during the Mesozoic era, some 70 to 100 million years ago. The river's current relentlessly carves through these deposits, revealing bands of rust-red and ochre, where fossils of ancient marine life whisper of a world long vanished. To the northwest, the land rises gradually, but beyond the horizon, the jagged peaks of the Rockies loom, their granite and gneiss roots formed deep in the Precambrian, some 1.7 billion years ago.

Fifteen miles downstream from the vantage point of Fort Benton, the Marias River joins the Missouri from the north. It was here, in June 1805, that Lewis and Clark confronted a pivotal choice--one that would determine the fate of their expedition. They had traveled for nearly a year, following the Missouri's muddy, turbid path westward, guided by the river’s color and flow. Now, two rivers converged, their waters mingling but distinct.

The northern fork, the Marias, carried a familiar brown hue, thick with silt and sediment stirred from the plains. The southern fork, however, ran crystalline and swift, its waters clear and cold, suggesting a source nearer to the mountains and their snowfields. Most of the men, relying on the river’s appearance, favored the north fork as the main Missouri. Yet Lewis and Clark trusted their instincts and observations, choosing the south fork. This decision was informed not only by the clarity of the water but also by their growing awareness of the landscape’s geological character.

Clark wrote in his journal on June 22, 1805: "We have found a river running from the south west which... appears to be the true Missouri." He and Lewis discerned that the clear waters bore the signature of mountain streams--filtered through glaciers and alpine soils, carrying less sediment than the plains rivers. This insight would lead them to the Great Falls of the Missouri, a series of cascading waterfalls that roar through the bedrock in a spectacle of water and stone.

The Great Falls themselves represent an extraordinary geological feature. Here the river plunges over a series of five major falls, dropping nearly 600 feet over ten miles. The falls carve through Precambrian igneous and metamorphic rocks, some of the oldest exposed formations in Montana. These ancient rocks, predominantly granite and gneiss, form the backbone of the Lewis Range and the adjacent continental divide. The falls create a natural barrier, forcing the expedition to portage their boats and supplies for nearly 18 miles around the treacherous rapids. This grueling effort underscored the immense physical challenges posed by the landscape.

In choosing the south fork, Lewis and Clark avoided the Marias River, which would have led them north into the Canadian Rockies--a far more arduous and uncertain route. Their decision saved them precious time. The Corps of Discovery had to cross the Rockies before the snows made passage impossible. As Lewis noted in his journal, "An error in our route at this time would be attended with the most serious consequences." The expedition pressed forward, reaching the headwaters of the Missouri at Three Forks, where the Jefferson, Madison, and Gallatin rivers converge, marking the true source of the great river.

The Missouri River below Fort Benton remains a remarkable stretch of free-flowing water--now part of the National Wild and Scenic Rivers System. It preserves an ecological corridor that supports species such as the paddlefish (Polyodon spathula), which glide beneath the surface, and the northern pike (Esox lucius), a predatory fish thriving in these nutrient-rich waters. Along the banks, stands of cottonwood (Populus deltoides) and plains cottonwood trees provide shade and nesting sites for birds like the western wood-pewee (Contopus sordidulus) and the belted kingfisher (Megaceryle alcyon).

From this vantage point, one can almost hear the distant thunder of the Great Falls, feel the chill of mountain air carried downriver, and imagine the meticulous observations of Lewis and Clark as they navigated these waters. Their journey was not only a feat of endurance but an exercise in acute naturalist observation, mapping a land of immense geological complexity and ecological diversity.

William Clark remarked on June 25, 1805, "The surrounding hills are a continued range of limestone and sandstone... the river runs through a narrow valley about a mile wide." This description captures the interplay between the sedimentary layers and the river valley, a dynamic shaped by erosion and uplift. Their notes form some of the earliest scientific records of this region’s geology and hydrology.

The choice made at the confluence of the Marias and Missouri rivers was no mere navigational guess. It was a calculated judgment grounded in close study of water color, flow, surrounding topography, and the subtle clues revealed by the landscape. It was a moment when observation and experience intersected, steering the expedition toward success rather than failure.

Standing here today, one senses the slow, immense forces that formed this landscape, from the ancient seas that laid down layers of sediment to the sharp uplift of the Rockies and the relentless sculpting by glaciers during the last Ice Age. All these factors converge to shape the Missouri River’s path--the river that guided the first American explorers west.

See also

• Lewis and Clark Trail at Fort Benton, Chouteau County
• Lewis and Clark National Historic Trail at Loma, Chouteau County
• Land of Many Uses at Shonkin, Chouteau County