Idaho Utah
Left: Bear Lake isn’t as warm as Caribbean waters, but thanks to all the limestone-rich rocks in the area, it’s just as blue. Right: Mud Lake, at the north end of Bear Lake, acts as a sediment trap for the Bear River, helping keep Bear Lake’s waters an ethereal blue.
Peering down into the turquoise water of Bear Lake, on the border between Idaho and Utah, one might suspect a change of flight plan. You don’t need a trip to the tropics, however, to swim in a sea of aquamarine. Bear Lake is nicknamed the Caribbean of the Rockies for good reason—light reflecting off minute particles of calcium carbonate suspended in the water give it a beautiful blue-green hue.
Bear Lake lies in a fault-lined valley, through which the Bear River flows out of the Uinta Mountains on its way to its terminus in Utah’s Great Salt Lake. This is a half-graben valley, which means it is bounded by a fault along only one side; a full graben is a depressed block of land enclosed on two sides by parallel faults. The valley was not carved by the river—rather, the river was captured by the valley as the fault deepened over time.
The freshwater lake as we know it formed around 150,000 years ago, filling a basin once occupied by a shallow, salty sea. After the sea evaporated, runoff from the Uinta Mountains filled the basin. Around 8000 years ago, a series of movements along the faults on the eastern side of the lake resulted in its present shape and size; Bear Lake covers just over a hundred square miles. This fault system remains active today, slowly dropping the eastern floor of the lake, where depths reach only ninety or so feet.
The region is dominated by limestone deposits, laid down during the Mesozoic Era. Erosion of these layers enriches the water of Bear Lake with calcium carbonate, which reflects blue light, giving the lake its preternatural color. Phosphorus is also abundant, and these unique water conditions have given rise to an unusually high number of endemic species of fish found only in Bear Lake, including the Bear Lake strain of the Bonneville trout, the Bear Lake whitefish, and the Bear Lake sculpin. Several unique species have gone extinct in the last few decades because of manmade changes in the lake’s hydrology.
Fossilized ripples preserved in a rock along Bear Lake’s shoreline hint at the area’s long history as an inland sea. The ripples were sculpted into mud by water. The mud then hardened into rock, preserving the pattern.
It’s hard to miss the brilliant turquoise-blue lake between southeast Idaho and northeast Utah. Salt Lake City, Utah, lies a hundred miles to the southwest.
