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The Snake River Plain-Yellowstone Hot Spot and its Effect on Drainage Patterns
Over the last seventeen million years, a topographic uplift or plateau, like the Yellowstone National Park area today, has moved northeastward across Idaho (malde, 1991; Pierce and Morgan, 1992). This northeastward migration is caused by the southwestward movement of the North American tectonic plate over a fixed hot spot, or area of production of molten rock or magma. The passage of the topographic uplift associated with the hot spot produced a northeast-progressing highland, from which streams drained to the north and south. Thus, the Continental Divide, at the headwaters of the Snake River, has migrated eastward across eastern Idaho. Streams which presently drain toward the Snake River Plain, from the north and south, formerly drained away from the volcanic plateau.

Simplified geologic map of the central and southern Rocky Mountains. The Oregon Trail crossed the Continental Divide at the south end of the Wind River Mountains, and headed west across the Green River Basin to the Idaho-Wyoming Thrust belt (heavy lines) and the Snake River Plain.From King (1977). Used by permission of Princeton University Press. Click on image for a larger view

Subsidence of the plateau in the wake of passage of the Hot Spot allowed the Snake River to carve its west-flowing course along the southern margin of the lava field, and resulted in capture of streams which formerly drained south and east, away from the highland. Marsh Valley, among others, formerly drained southward toward the Lake Bonneville Basin, and has only drained northward through Portneuf Narrows since the Hot Spot passed to the northeast of Pocatello several million years ago. It is as if the west-flowing Snake River, over the last few million years, has eaten its way eastward, capturing the water of drainages to the north, east, and south, and, now, with the help of irrigation systems less than 100 years old, spreads that water on the fertile soil of the Land of Famous Potatoes.