Geology of the Edson Fichter Nature Area

Geology of the Edson Fichter Nature Area

The Edson Fichter Nature area occupies meander bends of the Portneuf River south of Pocatello. The meanders offer a variety of fast and slow-moving river reaches, with associated fluvial and riparian ecological diversity. The meanders were built, before Pocatello was settled in 1870, by the Portneuf River as it reduced its gradient flowing northward toward the Snake River. The modern river channel is incised into boulder-bearing deposits of the Lake Bonneville Flood that were deposited about 17.4 ka (17.4 thousand years) ago. The boulders are up to several feet in diameter and most of them are black basalt eroded from the Basalt of Portneuf Valley which was erupted from vents near Bancroft in northern Gem Valley. The lower of two lava flows is dated at 430 ka. When the Utah Northern Railroad grade, which runs straight northwest, was built in 1878, several meanders south the Edson Fichter Nature Area were cut off from the river. The resulting reduced size of the river’s flood plain caused increased flood volume during the 1963 and 1964 rain-on-snow flood events, which led to damage downstream in Pocatello and to the construction of the flood-control channel by the U.S. Army Corps of Engineers in 1968.

By Paul Link, Department of Geosciences, Idaho State University