Roll the clock back 13,000 years, and every mile of US Route 89 in Utah between Spanish Fork and Logan Canyon sits beneath a cold, freshwater sea. The Great Salt Lake is the largest saltwater lake in the Western Hemisphere, but it’s a puddle compared to Lake Bonneville, which came before it. At roughly 20,000 square miles and nearly 1,000 feet deep in places, it was almost the size of Lake Michigan.
Lake Bonneville. Image credit: Wikipedia
How a Sea Was Born
The Bonneville Basin has no drainage to the ocean. Whatever water flows in can only leave through evaporation. Starting around 30,000 years ago, cooler temperatures slowed evaporation while snowmelt and rainfall poured in. The lake just kept rising. Mammoths, Western camels, and giant ground sloths roamed its shorelines. Their bones have been found in gravel quarries and lake sediments all along the Wasatch Front.
The Flood That Shook a Continent
About 14,500 years ago, Lake Bonneville reached a tipping point. The lake had been pressing against a natural dam of loose sediment at Red Rock Pass in southern Idaho, and one day it gave way. What started as a trickle became one of the largest floods in North American history. At peak discharge, roughly three times the average flow of the Amazon River roared through that gap, carving canyons, hurling boulders the size of cars across southern Idaho, and eventually draining into the Snake River and on to the Pacific. The lake dropped 350 feet in a geological instant. You can still see the high-water mark, a pale ring etched into the mountains above Salt Lake City, visible for miles.
What the Lake Left Behind
Bonneville Salt Flats. Image credit: Wikipedia
As Bonneville receded, it left behind a ring of hardened minerals and sediment, often referred to as a bench or terrace. This can be seen, even from long distances, as you drive through the Cache Valley.
It also left behind the Bonneville Salt Flats, a vast, mirror-flat expanse of white salt crust west of Wendover that is one of the most otherworldly landscapes in North America. The flatness comes from the lakebed itself: when the water finally evaporated, centuries of mineral deposits hardened into the surface you can walk, drive, and (if you’re feeling ambitious) attempt a land speed record on today.
Today’s Great Salt Lake, Utah Lake, and Sevier Lake are the surviving remnants of Bonneville. Sevier Lake is nearly always dry. Its main feeder, the Sevier River, is drained almost entirely for farm irrigation. Utah Lake remains freshwater because the Jordan River drains it into the Great Salt Lake. It has suffered from severe pollution for nearly a century, but conservation efforts have steadily improved its water quality. The Great Salt Lake is two to seven times saltier than the ocean and getting saltier because, with no outlet, water evaporates, but the salts stay behind.
The Lake That Makes Utah’s Snow
Here’s something most people don’t know: the Great Salt Lake is partly responsible for Utah’s legendary ski powder. When cold air masses roll across the lake’s warm surface in winter, they pick up heat and moisture that destabilize the atmosphere and squeeze extra snow out of storms passing over the Wasatch Range. The phenomenon is most dramatic at resorts like Alta and Snowbird in the Cottonwood Canyons. It’s why Utah markets itself as having the “Greatest Snow on Earth,” and it’s one more reason the lake’s decline is alarming.
A Lake in Trouble
Great Salt Lake, Antelope Island. Image credit: Vicky Derksen
The Great Salt Lake is shrinking fast, and the consequences reach further than most people realize. As the lakebed dries and exposes, it releases dust laced with arsenic, mercury, and lead that blows into Salt Lake City and the surrounding communities, threatening the health of nearly three million people.
Rising salinity is collapsing the food web. Brine shrimp and brine flies, the tiny tough creatures that thrive in this hypersaline environment, are struggling. That matters enormously, because those two species are the foundation of the entire ecosystem. They feed the ten million migratory birds that pass through on the Pacific and Central flyways every year, before their nonstop flights to wintering grounds. Take out the base of the food web, and the whole system collapses. The culprits are familiar: agricultural water diversions, a warming climate, and shrinking mountain snowpack.
The First People Here
As the lake receded, people moved in. They settled around the marshes, hunted the remaining megafauna, fished the remnant waters, and harvested brine flies and their protein-rich larvae from the lake’s edges. They lived in small family groups, sometimes sheltering in caves that Bonneville’s own waves had carved from limestone cliffs. Danger Cave, near the Bonneville Salt Flats, was occupied within a few thousand years of the lake’s retreat and contains one of the most significant archaeological records in North America. It holds layers of artifacts spanning more than 10,000 years of continuous human use.
These were the ancestors of today’s Ute, Paiute, Goshute, and Shoshone peoples, who lived in this basin for roughly 13,000 years before Euro-American contact. The Mormon pioneers arrived in 1847 and, within a generation, had diverted the rivers and streams these peoples had depended on for millennia, the same water diversions that are now finishing off the Great Salt Lake.
Places to See It for Yourself
Natural History Museum of Utah. Image credit: Vicky Derksen
Natural History Museum of Utah — The museum’s exhibits on Utah’s prehistoric landscape put Lake Bonneville in full context, from the megafauna that roamed its shores to the geology that shaped the basin. It sits on the University of Utah campus in Salt Lake City, right along the old Bonneville shoreline terrace. If you want to understand what you’re looking at before hitting the road, this is the place to start.
Bonneville Salt Flats — Take I-80 west toward Wendover and pull off at the Salt Flats exit. Standing on that white crust, you’re standing on the floor of a prehistoric sea. It’s flat enough to see the curvature of the Earth.
Great Salt Lake State Park — Located on the south shore of the lake, just off I-80 west of Salt Lake City, this is the easiest place to actually touch the water. The visitor center has displays on the lake’s history and a tank of live brine shrimp. From the viewing deck, you can see the old Bonneville shoreline terraces on the mountains across the water.
Antelope Island State Park — A causeway leads out to the largest island in the Great Salt Lake, where bison still roam, and the views of the surrounding basin are sweeping. It’s one of the best places to see the old Bonneville shoreline terraces on the distant mountains and to get a feel for just how vast the lake once was.
Spiral Jetty — Robert Smithson’s famous earthwork sits on the northeastern shore of the Great Salt Lake near Rozel Point, and it may be the most visually striking place to contemplate Bonneville’s legacy. The 1,500-foot coil of black basalt rock extends into reddish, hypersaline water. As lake levels have dropped over the decades, the jetty has gone from submerged to fully exposed, a living record of the lake’s decline. Getting there requires a dirt road, but it’s worth it.
Bear River Migratory Bird Refuge — Located northwest of Brigham City near the Great Salt Lake, this is the place to witness what Bonneville’s legacy makes possible: millions of migratory birds fueling up on brine shrimp and brine flies before continuing their journeys. Bring binoculars.
The Bonneville Shoreline — You don’t have to go anywhere special to see this one. While driving through the Cache Valley, look at the mountains. That pale horizontal band running along the foothills above the valley floor? That’s the old waterline, the edge of a lake that covered everything below it.
