Are Lake Bonneville's Shorelines Level?

This investigation examines whether the ancient shorelines of Lake Bonneville—an enormous prehistoric lake that once covered parts of Utah and neighboring states—are truly level as water surfaces normally are. By measuring and analyzing elevations across multiple locations corresponding to the well-preserved Bonneville and Provo shoreline levels, significant elevation differences were found, indicating the ground beneath the lake was depressed under the water's weight. This phenomenon, driven by Earth's crustal deformation (isostasy), provides insight into geological processes and demonstrates that ancient lake shorelines do not necessarily lie on a perfectly flat plane.

High-Level Overview

Lake Bonneville, a vast prehistoric lake that existed about 16,000 years ago in what is now Utah and surrounding regions, left behind visible shoreline terraces marking its highest extents. These terraces are natural wave-cut benches that provide evidence of water levels during the lake’s existence. The fundamental expectation is that ancient shorelines formed by water should all lie at the same gravitational potential—essentially the same elevation after correcting for Earth’s curvature. This analysis tests that assumption using barometric altimeters at multiple sites across the Bonneville basin to see if these ancient water marks are truly level. The findings not only challenge the simple notion of level shorelines but also illustrate the Earth's crustal response to large water masses, an important concept for fields such as geology, geomorphology, and environmental science.