Pleistocene Lakes of North America: The Vanished Inland Seas

Drive across the Great Basin today and you cross a landscape of dry valleys, white salt pans and pale benches cut into the hillsides. It is hard to imagine, but for much of the Pleistocene these basins held enormous freshwater lakes. Cooler temperatures and a wetter climate, combined with meltwater from mountain glaciers, filled the closed valleys of Nevada, Utah and eastern California with water hundreds of feet deep.

Why the lakes formed

Geologists call them pluvial lakes — lakes that owe their existence to a rainier, cooler climate rather than to a connection with the sea. During glacial maxima, less evaporation and more precipitation allowed water to pond in basins that have no outlet to the ocean. When the climate warmed at the end of the last Ice Age, roughly 10,000 to 15,000 years ago, evaporation overtook inflow and the lakes shrank, in many cases to nothing.

The two giants: Bonneville and Lahontan

Two lakes towered above the rest. Lake Bonneville, in what is now Utah, covered more than 20,000 square miles at its peak and left the Great Salt Lake, Utah Lake and the Bonneville Salt Flats as its diminished heirs. To the west, Lake Lahontan spread across northwestern Nevada in a branching, many-armed shape, leaving Pyramid Lake and Walker Lake as its survivors.

Reading the shorelines

The clearest evidence of these vanished seas is written on the mountainsides as horizontal terraces — old shorelines where waves once cut into the slope. They ring the ranges like bathtub rings, each level marking a period when the lake stood at one height long enough to carve a bench. From these strandlines geologists reconstruct how high the water rose and how quickly it fell.

• Wave-cut terraces mark long-standing lake levels.
• Gravel bars and spits show where currents moved sediment.
• Tufa towers — porous carbonate mounds — precipitated underwater and now stand exposed.

Why they still matter

The Pleistocene lakes are more than scenery. Their fine-grained lakebed sediments store a record of past climate, their shorelines help calibrate how the land has risen since the weight of water was removed, and their flats and playas remain some of the most striking landscapes in North America. To walk an ancient strandline is to trace, with your feet, the edge of a lake that vanished before written history began.