Document Type

Thesis

Date of Degree Completion

Spring 2001

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Lisa L. Ely

Second Committee Member

Carey Gazis

Third Committee Member

James Hinthorne

Fourth Committee Member

Karl Lillquist

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.

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