...the End of the Northwest Passage?

By editor

Dillon, Beaverhead County, Montana

On August 17, 1805, Meriwether Lewis paused at the confluence of the Beaverhead River and its tributaries in southwest Montana, a place where the waters of the Jefferson and Beaverhead rivers meet. He recorded a remarkable observation in his journal: "I do not believe that the world can furnish an example of a river running to the extent which the Missouri and Jefferson's rivers do through such a mountainous country and at the same time so navigable as they are." This statement captures not only his astonishment at the natural waterways but also an unspoken marvel at the underlying forces that shaped this landscape over countless epochs.

To understand Lewis’s amazement, one must consider the deep-time processes that formed these valleys and rivers. The broad, flat valleys of Beaverhead, Big Hole, and Red Rock in southwest Montana exist because the earth’s crust here has been stretching apart for nearly four million years. This extension, a geological activity known as crustal rifting, fractures the bedrock along faults. The blocks of crust trapped between these faults have subsided, forming the down-dropped valleys called basins, while the blocks on the sides have been uplifted into mountain ranges. This topography produces gentle valley floors that rivers can follow with minimal resistance, creating waterways that flow calmly and steadily enough to be considered navigable, even amid the Rocky Mountains.

The Jefferson River, which Lewis admired, flows through such a down-faulted valley. Its placid surface and steady current owe their character to this geologic history, a fact invisible to Lewis but evident to the geologist centuries later. The river’s course is not a struggle against the land but rather a submission to the slowly sinking ground beneath it.

Yet, the story changes abruptly west of the Continental Divide. The divide itself, lying just west of these Montana valleys, marks a tectonic and topographical shift. While the valleys east of the divide owe their form to crustal extension, the mountains west of it rise from a history of crustal compression. The earth’s crust here buckled, folded, and fractured, thrusting rock layers upward to form steep ranges and narrow canyons. The rivers draining this western slope, such as the Salmon River, rush through deep gorges and drop precipitously in elevation, at times plunging hundreds of feet per mile. These swift waters carve rapids and waterfalls, rendering the rivers impassable to the types of boats Lewis and Clark carried.

Thomas Jefferson, who commissioned the Corps of Discovery, hoped to find a water route across the continent--a Northwest Passage--that would allow for easy navigation from the Missouri River all the way to the Pacific Ocean. Lewis and Clark’s journey was in part a quest to confirm this hope. But geography and geology conspired to frustrate it. When Lewis crossed the Continental Divide at Lemhi Pass on August 12, 1805, he looked westward toward the Salmon River and its canyon, and noted with sober realism, "the view was not such as to encourage any hopes of a speedy termination of the voyage."

The Salmon River, known to the Shoshone as the "River of No Return," lives up to its name. Its turbulent flow through steep granite walls made it impossible for Lewis and Clark to continue by watercraft. They had to abandon their canoes and proceed on foot and horseback, guided by Native peoples with intimate knowledge of the terrain.

The geological forces that shaped these two contrasting landscapes--the gentle valleys east of the divide and the rugged canyons west--are written in the rocks beneath our feet. The basin-and-range pattern of southwest Montana formed during the Cenozoic Era, beginning roughly four million years ago, as the crust stretched and fractured. Sedimentary and volcanic rocks laid down in earlier periods were broken and shifted, creating the flat floors of valleys such as the Red Rock Valley and the Tendoy Mountains. This extension caused the rivers, including the Jefferson and Beaverhead, to seek out the lowest paths, flowing gently through vast basins.

In contrast, the mountains west of the divide rose during the Mesozoic Era, about 250 million years ago, when compressive forces folded and thrust-faulted sedimentary rock layers, producing steep slopes and narrow valleys. The Salmon River’s narrow canyons are a direct consequence of this intense uplift and folding.

In this way, the geology of the region determined the fate of exploration and travel. The rivers east of the divide invited navigation and transport, while those to the west formed barriers, demanding different modes of travel and knowledge. This sharp contrast in landscape and river behavior meant that the Northwest Passage Jefferson envisioned was more an idea than a reality.

As Lewis’s journal entries reveal, the explorers were keenly aware that nature was dictating their progress. Lewis’s reflection on the rivers’ navigability reveals a mind attentive to the earth’s subtle forms, even if he lacked the terminology of modern geology. His voyage was shaped by these natural features, not merely by human ambition.

Later geologists confirmed the lessons Lewis observed. Geologist Arthur Lakes, writing in the late 19th century, emphasized the importance of these structural differences: "The great mountain ranges of the West are not all alike. Some are the result of stretching and sinking, others of folding and thrusting, and these differences control the rivers and valleys we see." Lakes’ insight underscores how the region’s geological history shaped the routes available to early explorers and settlers.

Today, standing at the forks of the Beaverhead River, one can see the broad valley floor, the calm waters reflecting the sky and the rugged mountains beyond. The landscape preserves the story of deep geological time and the ambitions of men who sought to cross a continent by water. The rivers that Lewis praised for their extent and navigability owe their character to the slow but relentless forces of crustal extension, while the mountains west of the divide continue to guard their secrets in steep canyons and roaring rapids.

The dream of a continuous water route ended here, not from lack of will or skill, but because the earth itself would not yield its shape to human desire. In Lewis’s words, the journey required "horses, guides, and a different route." The Northwest Passage was found not in boats, but in the patience to learn the land’s contours and the humility to follow where it led.

See also

• ...the End of the Northwest Passage? at Dillon, Beaverhead County
• Camp Fortunate at Dillon, Beaverhead County
• Reliable Landmarks at Dillon, Beaverhead County