Earth and Planetary Sciences ETDs

Yehouda Enzel

Publication Date

10-9-1990

Abstract

A physical link between anomalous, present-day atmospheric circulation patterns over the North Pa1cific Ocean, extreme storms in southern California, the largest floods of record and lake stands in the Mojave River watershed in the Mojave Desert is demonstrated by analyzing hydrologic and climatic data. This link is then used as a modem analog to interpret hydroclimatic conditions during the latest Quaternary recorded in lake deposits in the Silver Lake playa, the terminal basin of the of the Mojave River.

The Mojave River filters out small to medium floods by discharge loss through infiltration into the alluvial aquifer and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous atmospheric conditions. During eight years of the 20th century storms: were vigorous enough to produce large flood events that were able to overcome the1 filtering-by-infiltration mechanism of the river, to reach the terminal playa and to produce shallow lakes. Each of these floods was due to precipitation with a durati1on of one or two days resulting in rainfall of greater than +2 standard deviations from the mean monthly precipitation in the headwaters of the Mojave River. These precipitation events occurred in response to extreme southerly displacement of both the winter storm activity and the Polar jet over the eastern North Pacific, directing subtropical moisture into southern California. Analyses of sea-level pressure (SLP) for the eight months with lake­building floods revealed that a) the eastern North Pacific subtropical high weakened, giving way to an anomalously low pr1essure system along the west coast of the United States, b) the central North Pacific winter low shifted to the east and south, c) there is a tendency for a 'split' Akutian low with higher than normal SLP in the Aleutian and an anomalously low SLJP over Kamchatka.

The historic lake-building storms and floods are relatively rare; however, they control the daily to decadal hydrological records of the Mojave River hydrologic system and probably indicate a different subgroup of extreme events within the data. Simulations of a late Holocene lake recorded in sediments in and around Silver Lake playa are based on the present-day hydrology of the Mojave River and its terminal playas and also on geological boundary conditions. The results of these simulations reveal that a perennial lake in the terminal playa will be produced only if present­day floods, with recurrence intervals greater than 20 years, occurred almost every year. The number of floods needed to produce a permanent lake is found to be insensitive to reduction in evaporation. Such perennial lakes occupied Silver lake playa for a few decades during at least three episodes in the Holocene. The two youngest episodes were dated to 390±90 and 3620±70 years B.P., corresponding to the 'Little Ice Age' and the Early Neoglaciation, respectively. The lakes imply a) greater than present moisture input into the Mojave River watershed and b) that this moisture was able to reach the terminal playas. This implies that more high­frequency, high-intensity storms occurred in the Mojave River headwaters during the late Holocene episodes. Atmospheric circulation patterns, similar to the modem anomalous ones, persisted and caused these storms and thus the Holocene lakes.

Geological and botanical evidence from other parts of the Great Basin and southern and central California indicate that the two late Holocene episodes, which were characterized by frequent present-day anomalous climatic conditions, influenced large portion of the western coast of North America and the North American deserts.

The modem anomalous atmospheric circulation pattern, which shifted the winter storm track to its southernmost position along the west coast of North America, is similar to simulation results for the last glacial maxima. This observation provides a possible scenario for atmospheric circulation that controlled the building and maintaining of the deeper and larger Lake Mojave, the late Pleistocene-early Holocene predecessor of Silver Lake playa, as well as other pluvial lakes in similar latitudes in North America.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Stephen G. Wells

Second Committee Member

Roger Yates Anderson

Third Committee Member

Leslie D. McFadden

Fourth Committee Member

Robert Balling

Fifth Committee Member

Richard Heggen

Project Sponsors

the U.S Department of Interior, the New Mexico Water Resources research Institute, the Graduate Students Association Research Allocation Committee, the American Water foundation

Language

English

Document Type

Dissertation

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