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History of the Great Salt Lake: Part 1, Lake Bonneville

Updated on November 7, 2012

Ancient Lake Bonneville, Outlined in Light Blue

When you look around at the mountains surrounding Salt Lake City, Utah (in the United States of America), you’ll notice a few shelves or benches high above the buildings and houses of the valley. Those benches were once beaches, of a lake that occupied the area between 30 and 14 thousand years ago. At the time it existed, there was no name given it, as far as we know. But today, we call it Lake Bonneville. The lake was run-off from melting glaciers of the Ice Age. It built up through the years because there were no rivers to drain from it. The lake rested for a time at 4.4 thousand feet above sea level (about 23,000 years ago), long enough to create beaches or benches at that level. But later it continued to rise, creating the highest benches at around 5 thousand feet.

Around 17,000 years ago, the lake’s level reached high enough to breach the summit at Red Rock Pass in southern Idaho, the state to the north of Utah. The water began to spill over, and because of the volume of water and the added force due to the crumbling earth below the water, the breach quickly widened and turned into a catastrophic flood that carved out canyons along the Snake River and carried boulders the size of small houses far from their native origins. The following link shows some of those boulders and the aftermath of that flood.

After the rush of water had subsided, the lake leveled out during the next 600 years at about 4.7 thousand feet, creating more benches. As more water leaked out and evaporated while the climate warmed up, the size of the lake continued to dwindle until it reached the size we see today.

As the lake shrunk, its minerals became more concentrated. Rivers flowing into the lake brought even more metals. Today, it still ingests about two million tons of salts per year from its four major contributing rivers.

Because of the high concentration of salt, one can float in the lake without much effort.

Figure 1
Figure 1
Figure 2
Figure 2

Here are some photos I recently took of the mountains around Salt Lake City. In Fig. 1, the bench appears to be rising toward the right, but this is because I was below the level of the bench, and an arm of the mountain is coming out into the valley. The yellow lines represent the spine of that arm coming out at us. The yellow arrow shows where the uppermost bench is.

Fig. 2 shows a beach ridge that has dried up. Subsequent rains from the hills above had spilled over the ridge, creating erosion fingers. Houses were recently built on that bench. This photo shows where the bed of the lake was at a little higher elevation, hence the bench is not far above the valley floor. In this photo, we are looking south-eastward, which puts Utah Lake to our right, and the Great Salt Lake to our left or behind our left shoulder. A river (the Jordan River) flows from Utah Lake into the Great Salt Lake, Making this a higher elevation than the bed of the current Great Salt Lake.


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