Bear Lake, straddling the border between Utah and Idaho, is known for its intense turquoise-blue color, often called the “Caribbean of the Rockies.” The lake’s vibrant hue is caused by the refraction of light off suspended calcium carbonate deposits in the water. While it is a popular destination for recreation, its formation story begins with the deep-seated movement of the Earth’s crust.
The Role of Tectonic Faulting
Bear Lake is classified as a tectonic lake, meaning its basin was created directly by the movement of the Earth’s crust. The region lies within the active Intermountain Seismic Belt, characterized by horizontal extension, or stretching, of the crust. This stretching causes large blocks of the Earth’s surface to move vertically along fractures known as normal faults.
The mechanism involves the Bear Lake Fault Zone, a major system of normal faults running along the eastern side of the valley. As the crust pulled apart, the land block on the east side was uplifted, forming the steep Bear River Range. Simultaneously, the block of land beneath what is now the lake dropped down, creating the depression.
Formation of the Graben Basin
The result of this localized crustal stretching and vertical movement is a geological structure known as a half-graben basin. A half-graben is a depressed block of land that sinks relative to the surrounding rock. In the case of Bear Lake, the steep East Bear Lake Fault forms the pronounced eastern boundary.
The ongoing subsidence of this central block over millions of years created the deep, asymmetric trough that defines the lake’s shape. The lake floor and underlying sediments dip steeply eastward toward the main fault line. This explains why the lake is deepest along its eastern shore, reaching a maximum depth of approximately 208 feet, with a more gradual slope along the western side. The faulting continues today, slowly deepening the basin.
Ancient Lake Bonneville and Hydrological Filling
The lake’s current age is estimated at over 250,000 years, making it one of the oldest continually wet lakes in North America. During the Pleistocene Epoch, the basin’s hydrology was heavily influenced by the massive ancient Lake Bonneville, which covered much of the Great Basin.
The Bear River was a significant factor in filling the basin. About 60,000 to 50,000 years ago, volcanic activity redirected the Bear River southward, causing it to flow toward the Lake Bonneville basin. During high-water stages of the Pleistocene, Lake Bonneville levels were high enough to connect with and contribute water to the nearby Bear Lake basin.
The river flowed into the Bear Lake basin at various times throughout its history, causing the lake level to rise significantly. However, for most of the last 11,000 to 15,000 years, the lake was isolated from the main Bear River channel by a sediment berm that formed on the north end of the valley. During this isolation, the lake was primarily sustained by groundwater and small local streams, creating unique chemical conditions that allowed several endemic species to evolve.