Floods

Floods and the Portneuf

Flood events are characteristic of rivers everywhere, and the Portneuf has had its share of flood events during the past century. Floods may be triggered by severe storms, by rapid melting of winter snow, and by ice jams that are created during spring thaws. The record of flood events in the lower Portneuf is limited, but old photographs document a flood that probably took place early in the 1900’s.

(Photo courtesy of the Bannock County Historical Museum)

More recently, floods in both 1962 and 1963 covered large areas in the valley and in the City itself, causing extensive damage.

This photo shows the extent of flooding in 1962. This view looks south towards the Portneuf Gap. (Photo courtesy of the US Army Corps of Engineers)

In response to these floods, the US Army Corps of Engineers constructed the concrete channel that now encloses the Portneuf through a section of Pocatello.

This photo shows flooding that occurred in 1963. The large expanse of water in the center of the image covers the football field behind Irving Junior High School. (Photo courtesy of the Bannock County Historical Museum) Above average snowfall, and the resulting snowpack in the mountains, resulting in some record high daily flows in the Portneuf in spring 2006. Isolated areas along the lower Portneuf River flooded, and some of the old oxbows (bends in the old river channel that were cut off by the Union Pacific railroad tracks) were filled with water for the first time in a number of years. The photograph to the right shows some of these oxbows just south of the Cheyenne crossing on the south end of Pocatello. The active river channel runs just above (west) of the railroad tracks; the brown color of the water reflects the large amount of sediment in the water. The water-filled areas below the tracks have no surface connection to the active channel, but filled with water as the water table rose. The highway pond, further south towards the Portneuf Gap, also filled to its highest level in many years as groundwater rose in response to the heavy spring runoff. This pond, the site of an old gravel mining operation, is thought to connect directly to groundwater. As a result, contamination of this area poses a significant threat to Pocatello’s drinking water supply. The photograph to the right was taken towards the north; I15 is visible in the upper right corner, and the Union Pacific tracks are to the left of the pond.