A small look at the big picture: linking geopotential height anomalies to paleofloods on the Snake River, Idaho and Oregon

 
Gwendolyn Rhodes
December 2000

Abstract

A combined paleoflood and flood hydroclimatology study on the Snake River in Idaho and Oregon suggests a link between floods and patters of geopotential height anomalies over the North Pacific Ocean. Examination of the paleoflood record in two 4-m (12 ft) tall terraces along the Hells Canyon reach of the Snake River in Idaho and Oregon shows evidence of at least twenty-two late-Holocene extreme floods that occurred approximately 5,000 years. The ages of paleoflood deposits at these sites fall into two time periods, from 5130 ± 40 to 1960 ± 40 yr BP and from 320 ± 40 yr BP to post-AD 1950 with one flood deposit in between. The significant reduction in the number of deposits between these two time ranges suggests a hiatus in floods that were able to leave slackwater deposits above the 3-m (9 ft) height.

Examination of a 52-year record of recent flood-climate variability shows a consistent pattern of northeast trending negative geopotential height anomalies over the northern Pacific Ocean prior to extreme winter floods on the Snake River. The pattern of northeast-trending negative geopotential height anomalies in winter may be responsible for the floods that generated the 5,000-year long paleoflood record on the Snake River.

Since the Snake River drains a climatically diverse region, understanding the nature of past floods on the river enhances a larger effort to understand the links between flood hydroclimatology and flood frequency in the northwestern United States.

Full Thesis (PDF format)