While mapping the American Falls region, we found evidence that contributes to the middle Pleistocene to Recent history of the Snake River, and indirectly to the history of overflow of Lake Bonneville. Middle Pleistocene to recent rocks in the valley are mainly lacustrine and fluvial silts and clays, with some sand, gravel, basalt, and a few thin tuff beds. The formation of terraces can be correlated with events both up- and downstream.
The Snake River was at least once, and possibly twice, dammed and diverted by eruptions of basalt, resulting in the formation of lakes and deposition of lacustrine beds. A rather flat-lying, thin, but persistent gravel at the base of one lake bed formation may represent a glacial period, possibly Illinoian, during which the Snake River had a large volume.
Overflow of water from Lake Bonneville into the Snake River system, by way of the Marsh Creek-Portneuf valley, laid down a deltaic-fluvial deposit here named the Michaud Gravel. At this time the Snake River, greatly augmented by Lake Bonneville overflow, began to cut channels through and around a lava dam. Terraces between Aberdeen, American Falls, and Pocatello were formed during the existence of the lake in which the Michaud Gravel was deposited and by fluvial processes after drainage of the lake. At one stage in the downcutting, bars of huge basalt boulders were built across the mouths of abandoned spillways.
Radiocarbon dating and geologic evidence from the area between Preston and Soda Springs, Idaho, suggest that basalt flows diverted the Bear River into Lake Bonneville, perhaps causing it to overflow. This diversion probably occurred about 33,000 years ago. This dating accords with events in the American Falls region.
