Alaska rivers flow through mountain range

Alaska’s geography contains a remarkable natural feature that makes life in the Interior more accessible and practical. Because of a unique river corridor, residents can enjoy easier transportation routes, receive goods such as affordable bananas, and maintain communities in the challenging subarctic environment. The Nenana River, which begins on the southern side of the Alaska Range, turns around and travels northward through the mountains. Along its path, it created a broad, low-elevation passage that later became an ideal route for the Alaska Railroad and the Parks Highway.

The Unusual Path of the Nenana River

“Normally, a mountain range acts as a strong barrier,” explained Don Triplehorn, a professor emeritus at the University of Alaska Fairbanks who has long studied and explored a wide range of natural topics. He noted that the Nenana River follows a very unusual course.

The river begins by flowing southward, following the natural downhill direction expected of a river, but then changes direction, moving west and eventually north past Denali. According to Triplehorn, this kind of behavior is extremely uncommon.

Other Rivers Crossing the Mountains

The Nenana River is not the only major river that passes through the Alaska Range. The Delta River follows a similar pattern, beginning south of the mountains before cutting northward across the range.

“These waterways cross one of the tallest mountain systems on Earth,” Triplehorn explained. The wide, gentle valleys created by these rivers have become important natural pathways used for roads, pipelines, communication facilities, and the movement of people, wildlife, and vegetation.

How the Rivers Formed Their Unusual Courses

Why do these rivers appear to ignore the natural barrier of the Alaska Range? Triplehorn supported a theory proposed by his friend Tom Hamilton, a geologist from Anchorage. The idea suggests that massive glaciers moving northward across the mountains carved out the broad valleys where the rivers now flow.

The Alaska Range formed roughly 6 million years ago. During the last 2 million years, Earth experienced repeated ice ages, and the region south of Fairbanks was once covered by enormous glaciers resembling the modern Greenland ice sheet. Because moisture generally arrived from the south, the thickest part of the ice shifted in that direction. This allowed glaciers to move away from the high point, and in certain locations they traveled northward, cutting deep and wide valleys across the buried mountain landscape.

As the glaciers eventually retreated, streams formed from melting ice followed these newly created valleys. The waterways that became the Nenana and Delta rivers continued flowing north even after the glaciers disappeared, preserving the routes shaped by the ancient ice.

Different Explanations and Scientific Debate

Scientists have proposed several explanations for why the Nenana and Delta rivers cross the Alaska Range. However, Triplehorn pointed out that traditional ideas about rivers flowing around mountain barriers do not fully explain the unusual paths of these two waterways.

Regardless of the exact cause, these rivers and their valleys have had an enormous impact on human history and settlement in Alaska.

The Importance of the Mountain Passes

Without these natural corridors, life in Interior Alaska would look very different. Triplehorn suggested that Fairbanks might never have developed, because communities north of the Alaska Range may have instead formed mainly along the Yukon River and its tributaries.

Unlike regions such as Colorado, where major highways require tunnels through high mountains, Alaska’s broad passes across the Alaska Range have provided open travel routes for thousands of years. These passages have allowed people, animals, oil, vehicles, and even trucks transporting bananas to move through the mountains with relative ease.

Geography Shaping Human History

The story of the Nenana and Delta rivers demonstrates how geology and geography can influence the development of human societies. Natural features created millions of years ago continue to shape transportation, settlement patterns, and daily life in Alaska today.

Since the late 1970s, the director of the Geophysical Institute at the University of Alaska Fairbanks has supported the publication and free distribution of this science column to news organizations. In 2014, Ned Rozell marked his 20th year as a science writer for the Geophysical Institute. This article was originally published in 2007.