CONTENTS

[Note to readers: Each first-rank text heading links to a separate HTML page. Figures and tables, linked here and in the text, open in separate windows.]

Abstract

Introduction

Data and Methods

Results and Discussion

Mississippi River

Mississippi River at St. Louis

Mississippi River at Vicksburg

Rio Grande

El Niño/Southern Oscillation (ENSO)

Conditions during ENSO Events

ENSO Events and Discharge Records

Summary and Conclusions

Acknowledgments

References Cited

FIGURES

1. Map showing States in the Central and Eastern United States, the Gulf of Mexico, the Mississippi River and its major tributaries, and the Rio Grande.

2. Line graph showing ¹⁸O values for Globigerinoides ruber plotted against depth in piston core EN32-PC6 from the Orca Basin, Gulf of Mexico.

3. Line graphs showing variation in ¹³C and ¹⁸O for Globigerinoides ruber plotted against radiocarbon years for Ocean Drilling Program core 619A from the Pigmy Basin, Gulf of Mexico.

4. Chart summarizing individual El Niño (warm phase) and La Niña (cold phase) events over calendar years 1877 to 1999.

5-9. Maps showing gaging stations and dams and graphs showing mean annual discharge through time at gaging stations along the--

5. Illinois River.

6. Missouri River.

7. Ohio River.

8. Arkansas River.

9. Red River.

10. Graphs showing mean annual discharge through time at three gaging stations: Marseilles on the Illinois River, Hermann on the Missouri River, and St. Louis on the Mississippi River.

11. Graphs showing mean annual discharge through time at four gaging stations: St. Louis on the Mississippi River, Metropolis on the Ohio River, Van Buren on the Arkansas River, and Vicksburg on the Mississippi River.

12. Four maps of the United States showing Palmer Drought Severity Index reconstructions based on tree-ring studies for 1963, 1964, 1966, and 1967.

13. Graph comparing discharge record of the Mississippi River at Vicksburg, Miss., with a proxy for wet and dry conditions derived from tree-ring studies in the Great Plains region (eigenvector 1 of Meko, 1992).

14. Map showing gaging stations and dams and graphs showing mean annual discharge through time at three gaging stations along the Rio Grande.

15. Two block diagrams showing sea-surface temperatures and the position of the thermocline in the equatorial Pacific during normal conditions and during El Niño conditions.

16. Graphs showing mean annual discharge for the Mississippi River at St. Louis and Vicksburg and the Ohio River at Metropolis and showing El Niño and La Niña events.

17. Graphs showing mean annual discharge for the Rio Grande at Embudo and Presidio and showing El Niño and La Niña events.

18. Graph showing the mean annual Southern Oscillation Index (SOI) and the monthly SOI, 1870-1995.

19. Graph showing cross-spectral analysis of the mean annual discharge of the Rio Grande at Embudo, 1915-97, and the Southern Oscillation Index.

20. Graph showing cross-spectral analysis of the mean annual discharge of the Rio Grande at Presidio, 1930-97, and the Southern Oscillation Index.

21. Graph showing smoothed (3-year moving average) mean annual discharge of the Mississippi River at Vicksburg, Miss.

TABLES

1. Data from the Climate Prediction Center of the National Oceanic and Atmospheric Administration, 1950-99, used by Poore, Darling, Dowsett, and Wright (PDDW, this study) to identify El Niño and La Niña conditions.

2. Results of the Mann-Whitney U-test to determine if El Niño/Southern Oscillation extremes significantly affected flow at five gaging stations.

METRIC CONVERSION FACTORS

Multiply	By	To obtain
centimeter (cm)	0.3937	inch
cubic foot per second cfs	0.02832	cubic meter per second