U.S. Department of the Interior

U.S. Geological Survey

Climatic Fluctuations, Drought, and Flow in the 

Colorado River Basin

USGS Fact Sheet 2004-3062 version 2

August 2004

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Figure 1. Moisture sources to the Colorado River basin.

precipitation with elevation. Cold 

frontal systems produce substantial 

amounts of snow above about 5,000 

feet and rainfall at lower elevations 

in the Rocky, Uinta, and Wind River 

Mountains, which constitute the 

headwaters of the Colorado River 

and its principal tributary, the Green 

River. These storms build snowpacks 

that melt in the late spring, providing 

runoff to the Colorado River. Warm 

winter storms, which may tap 

moisture from the tropical Pacific 

Ocean, may produce rainfall on 

snowpacks, resulting in high runoff 

and floods on major rivers. The 

frequency and moisture content of 

frontal systems are strongly affected 

by atmospheric circulation patterns 

and sea-surface temperature (SST) 

of the tropical and North Pacific 

Oceans. 

Moisture delivered to the Colorado 

River basin during summer typically 

is a mixture of moist air from 

Introduction

Climatic fluctuations have profound effects on water resources in 

the western United States (fig. 1). 

In the arid and semiarid Southwest, 

climatic fluctuations affect many 

hydrologic characteristics of watersheds, including the quantity of base 

flow, the occurrence of large floods, 

and the timing of snowmelt runoff 

(Cayan and others, 1999; Stewart 

and others, 2004). Since the start 

of a persistent drought around the 

year 2000, inflows to Lake Powell 

on the Colorado River have been 

below average, leading to drawdown 

of Lake Mead and Lake Powell (fig. 

2), the primary means of storage 

on the river. Although the factors 

that caused and have sustained this 

drought have not been positively 

identified, it is possible to describe 

the broader climatic context within 

which it is occurring. This fact sheet 

provides a perspective on the current 

drought occurring in parts of the 

western United States, particularly 

in the Colorado River basin.

Sources of Moisture to the 

Colorado River Basin

Precipitation on the Colorado 

Plateau is biseasonal, having 

both winter and summer regimes 

(Hereford and others, 2002). In the 

headwaters, precipitation generally 

is evenly distributed across the four 

seasons, accumulating mostly in 

snowpacks. Moisture comes from 

several sources (fig. 1). Winter and 

spring frontal systems originating 

in the North Pacific Ocean, provide 

the largest and most important 

source of moisture. These largescale systems tend to carry moisture 

at higher levels in the atmosphere, 

with orographic effects of the mountainous West causing an increase in 

the Gulf of Mexico, the Gulf of 

California, and the eastern Pacific 

Ocean. Known as the “North 

American monsoon,” this moisture 

arrives at low levels in the atmosphere in July and August. Under 

the influence of strong heating and 

upslope flow, the moist air helps 

spawn thunderstorms that generally 

deliver high-intensity rainfall to 

elevations below about 7,000 feet 

and lower-intensity rainfall at 

higher elevations. Thunderstorms 

tend to be of small spatial extent, 

and although they may cause 

severe flash flooding locally, few 

floods are generated on the larger 

rivers in the region.

Warm-core systems (tropical 

depressions, tropical storms, and 

hurricanes) form in the eastern 

North Pacific Ocean off the west 

coast of Mexico. These storms 

rarely make direct landfall on the 

continental United States, tending 
Figure 2. Replicate photographs of Lake Powell at the confluence with the 

Dirty Devil River (entering from left). A. June 29, 2002. B. December 23, 2003. 

(Photographs by John C. Dohrenwend)

instead to dissipate over the ocean. 

Residual moisture from such storms, 

however, which can be considerable, 

is occasionally transported inland 

into the southwestern United States. 

This residual moisture, as well as that 

from tropical systems that occasionally transit the Southwest, produces 

precipitation that can generate large 

floods in the southern half of the 

Colorado River basin.

Oceans, Climate, and Drought

During the past 30 years, our 

understanding of the climatic importance of the oceans, particularly 

ocean temperature, has steadily 

improved. Initial research focused 

on the distant effects of the recurrent 

warming of the equatorial Pacific 

Ocean referred to as El Niño, which 

South American fishermen have long 

known to have an adverse affect on 

the coastal fisheries in Peru. El Niño 

is the warm phase of the sea-surface 

temperature component of a coupled 

ocean-atmosphere process called El 

Niño-Southern Oscillation (ENSO), 

which spans the equatorial Pacific 

Ocean. The atmospheric component, 

the Southern Oscillation, refers to a 

“seesaw” effect in sea-level pressure 

between the tropical Pacific and 

Indian Oceans. Reduced sea-level 

pressure in the Pacific combined with 

increased sea-level pressure in the 

Indian Ocean leads to a weakening 

in the trade winds over the eastern 

Pacific. This weakening enables 

warm water from the central equatorial Pacific to spread eastward and 

southward along the west coast of 

South America, creating the classic 

El Niño condition. Conversely, 

and about as frequently, the sealevel pressure in the Pacific Ocean 

increases while pressure in the Indian 

Ocean decreases, which causes trade 

winds to intensify over the eastern 

Pacific. When this occurs, equatorial 

upwelling of deep, cold water and 

cold water from the west coast of 

South America are pulled northward 

and westward from the coast into the 

eastern and central Pacific, producing 

La Niña. Thus, El Niño and La Niña 

are, respectively, the warm and cold 

phases of the coupled ENSO system. 

ENSO events typically last from 

6 to 18 months and, therefore, are 

the single most important factor 

affecting interannual climatic variability on a global scale (Diaz and 

Kiladis, 1992). ENSO has been 

linked to the occurrence of flooding 

in the lower Colorado River basin 

(Webb and Betancourt, 1992) and to 

both floods and droughts across the 

western United States (Cayan and 

others, 1999). Warm winter storms 

have been enhanced during El Niño, 

causing above-average runoff and 

floods in the Southwest, such as 

during 1982-1983. Not all El Niño 

events, however, lead to increased 

runoff in the Southwest. For 

example, during the 2002-2003 warm 

episode, runoff was below average in 

the Colorado River basin. Similarly, 

La Niña, is frequently, though not 

always, associated with belowaverage flow in the Colorado River. 

As a result, although ENSO exerts a 

strong influence in modulating wet 

versus dry conditions in many parts 

of the United States, the effect is not 

always the same in any given region. 

Some condition other than ENSO 

must also be influencing weather 

and climate patterns affecting the 

Colorado River.

In the mid-1990s, scientists identified another ocean temperature 

pattern, this one occurring in the 

extratropical Pacific Ocean north 

of 20oN (Mantua and Hare, 2002), 

which was named the Pacific 

Decadal Oscillation (PDO). The 

PDO varies or oscillates on a decadal

scale of 30-50 years for the total 

cycle; that is, much of the North 

Pacific Ocean will be predominantly though not uniformly warm 

(or cool) for periods of about 15 to 

25 years. During the 20th century, 

the PDO exhibited several phases, 

being warm along coastal southeastern Alaska from 1923-1943 and 

again from 1976-1998, and cool 

from 1944-1975. Since 1999, the 

PDO has exhibited higher-frequency 

fluctuations, varying from cool 

(1999-2001) to warm (2002-2004). 

Currently, the causes of the variaA.

B.
Figure 3. Time-series plot of the annual flow volume (in millions of acre-feet) for the Colorado River at Lee’s Ferry. Dashed 

line is the linear trend for the period. Vertical bars and shading delineate drought periods as defined using the Palmer 

Drought Severity Index for the climate divisions encompassing the upper Colorado River basin.

tions in the PDO are unknown and its 

potential predictability is uncertain. 

Recent research indicates that the 

PDO phase may be associated with 

decadal-length periods of above- and 

below-average precipitation and 

streamflow in the Colorado River 

basin (Hidalgo, in press) but, as with 

ENSO, such associations are not 

always consistent.

Most recently, researchers discovered that sea-surface temperatures in the North Atlantic Ocean 

exhibit a similar pattern of long-term 

persistence. The pattern, identified as the Atlantic Multidecadal 

Oscillation (AMO), refers to multidecadal warm and cool periods in 

ocean temperature each lasting 20 

to 35 years (Kerr, 2000; Enfield and 

others, 2001). Atlantic “cool” phases 

were observed from 1902-1925 and 

from 1964-1994, whereas a “warm” 

phase occurred almost continuously 

from 1926-1963. The Atlantic again 

entered a warm phase in 1996 and 

has remained so since. Such long 

periods of consistent temperature 

indicate that the AMO is much more 

persistent than the PDO. In addition, 

preliminary research indicates that 

these long periods of persistent 

Atlantic temperatures may be associated with the delivery of moisture 

to the United States. For example, 

Schubert and others (2004) found 

that during warm Atlantic phases, 

the easterly atmospheric flow from 

the Atlantic and Gulf of Mexico 

is shifted farther southward, and 

moisture delivery to the conterminous United States is diminished in 

certain seasons. In addition, several 

investigators have reported that warm 

Atlantic Ocean conditions appear to 

coincide with an increase in drought 

frequency over parts of the United 

States (Gray and others, 2003; 2004; 

McCabe and others, 2004). 

As our knowledge about the 

influence of the oceans on climate 

increases, so too does our awareness 

of the enormous complexity of the 

ocean-atmosphere system. Over time, 

scientists have learned that no single 

feature of the system (e.g., ENSO, 

PDO, AMO) explains all of the 

variations that occur in weather and 

climate. It is increasingly evident that 

the various features are interrelated 

in a complicated fashion. Their effect 

on climate depends on the direction 

and magnitude of their variations and 

possibly other factors that may not 

yet have been identified.

Flow in the Colorado River

Flow in the Colorado River varied 

significantly during the 20th century. 

Lee’s Ferry (fig. 1) is, by convention, taken to be the location that 

separates the upper-basin States of 

Wyoming, Colorado, Utah, and New 

Mexico from the lower-basin States 

of Nevada, Arizona, and California 

as documented in the Colorado 

River Compact of 1922. Calendaryear flow volumes (fig. 3) were 

combined from three data sets that 

were measured or estimated using 

different techniques. Consumptive 

water uses in the basin upstream of 

the stream-gaging station at Lee’s 

Ferry are not included in these data. 

The flow values represent the amount 

of water in the river at Lee’s Ferry. 

Thus, variability in the flow values 

is due both to climatic fluctuations 

as well as to changes in consumptive 

water use upstream from the Lee’s 

Ferry station. 

The primary data for the Colorado 

River at Lee’s Ferry were estimated 

or collected from 1895 through 

1962, one year before flow regulation began at Glen Canyon Dam. 

From 1895 through 1922, annual 

flow volumes at Lee’s Ferry were 

estimated by LaRue (1925, p. 108), 

and are distinguished from volumes 

measured at the Lee’s Ferry streamgaging station from 1923 through 

1962. 

From 1963 through 2003, flow at 

Lee’s Ferry is assumed to be approximated as the sum of flow volumes 

of the principal rivers flowing into 

Lake Powell. From 1950 through 

1962, comparison of these inflows 

with the flow measured at Lee’s 

Ferry indicated that inflow was, on 

average, 290,000 acre-feet per year 

less than the measured flow (about 2 

percent of the annual flow volume). 

Although this is well within the 

measurement error of stream-gaging 

stations, to account for this bias, 

the inflows to Lake Powell were 

adjusted using a regression equation 

calibrated on earlier years for the 

period 1963-2003. The time series 

of flow volumes (fig. 3) shows that 

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the average annual volume was 12.4 

million acre-feet (MAF) from 1895 

through 2003. This volume is less 

than the more commonly quoted 

annual volume of 15.0 MAF because 

these analyses do not include water 

that is consumptively used in the 

upper basin States. The trend in 

annual flow volume, which decreased 

by about 0.5 MAF per decade from 

1895 through 2003 (fig. 3), is due, 

in part, to upstream water use.

The period 1905-1922, which 

was used to estimate water production allocated under the Colorado 

River Compact, had the highest 

long-term annual flow volume in 

the 20th century, averaging 16.1 

MAF at Lee’s Ferry. The highest 

annual flow volume occurred in 

1984 (22.2 MAF), and the highest 

3-year average is 20.3 MAF for the 

period 1983-1985. The lowest annual 

flow volume is 3.8 MAF in 2002, 

followed by 3.9 MAF in 1934 and 

4.8 MAF in 1977. In the Lee’s Ferry 

record, which does not account for 

consumptive use upstream, flow in 

the early 21st century is the lowest in 

more than 100 years of observations, 

averaging only 5.4 MAF during the 

3-year period 2001-2003. The lowest 

previous 3-year average flows were 

7.3 MAF from 1954 through 1956 

and 8.0 MAF from 1933 through 

1935. 

Preliminary data from the U.S. 

Bureau of Reclamation (USBR), 

which include adjustments for 

upstream consumptive use from 

1906 through 2003, indicate that the 

lowest 3-year average flow at Lee’s 

Ferry, 9.4 MAF, occurred during 

water years 2000-2002. A water 

year begins on October 1 and ends 

on September 30 of the following 

calendar year. This was followed 

closely by water years 2001-2003 

(9.5 MAF). The USBR estimated 

flow for water year 2004 indicates 

that 2002-2004 will be the lowest 

3-year average flow in the observed 

record at 9.2 MAF. Before the recent 

drought years, the lowest observed 3-

year average flow reported by USBR 

was 9.8 MAF during the period 

1953-1955.

Long-Term Perspective 

on Drought Duration in the 

Colorado River Basin

 The length of observational 

records of climate and hydrologic conditions in the Colorado 

River basin is generally less than 

100 years. Researchers, however, 

generally prefer to assess episodes 

like drought, which have a multiyear 

character, using time frames of 

centuries rather than decades. One 

way to do so is to extend the observational records of climate using proxy 

records; that is, to use variables that 

are indicative of climate, such as 

annual bands in coral, lake sediment 

deposits, and tree-ring widths, for 

which much longer records exist. 

Analyses of tree rings have been 

used extensively to reconstruct the 

history of drought in the United 

States for the past 800 years. Treering reconstructions of precipitation 

in northern Utah (Gray and others, 

in press) indicate that, since 1226 

A.D., nine droughts have occurred 

lasting 15-20 years and four droughts 

have occurred lasting more than 20 

years. Moreover, tree-ring records 

indicate that some past droughts in 

the Colorado River basin persisted 

for several decades (Meko and 

others, 1995). Such findings from 

the tree-ring record, coupled with 

new findings about the connection between the AMO and drought 

frequency (McCabe and others, 

2004), suggest that the current 

drought could continue for several 

more years. Alternatively, the three 

droughts that affected the basin 

during the 20th century (Fig. 3) each 

lasted from 4 to 11 years, indicating 

that the current dry conditions could 

shift to wetter conditions at any time. 

Selected References 

Cayan, D.R., Redmond, K.T., Riddle, L.G., 

1999, ENSO and hydrologic extremes in 

the western United States: J. Climate 12: 

2881-2893.

Diaz, H.F., and Kiladis, G.N., 1992, 

Atmospheric teleconnections associated 

with the extreme phases of the Southern 

Oscillation, in Diaz, H.F., and Markgraf, 

V.A. (eds.), El Niño: Historical and 

Paleoclimatic Aspects of the Southern 

Oscillation: Cambridge, Cambridge 

University Press, p. 7-28.

Enfield, D.B., Mestas-Nuñez, A.M., and 

Trimble, P.J., 2001, The Atlantic multidecadal oscillation and its relation to 

rainfall and river flows in the continental 

U.S.: Geophys. Res. Let. 28: 2077-2080.

Gray, S.T., Betancourt, J.L., Fastie, C.L., and 

Jackson, S.T., 2003. Patterns and multidecadal oscillations in drought-sensitive 

tree-ring records from the central and 

southern Rocky Mountains: Geophys. 

Res. Let. 30:49-1:49-4

Gray, S.T., Graumlich, L.J., Betancourt, J.L., 

and Pederson, G.T., 2004, A tree-ring 

based reconstruction of the Atlantic 

Multidecadal Oscillation since 1567 

A.D.: Geophys. Res. Lett. 31: L12205, 

doi:10.1029/2004GL019932.

Gray, S.T., Jackson, S.T., and Betancourt, 

J.L., in press, Tree-ring based reconstructions of interannual to decadal-scale 

precipitation variability for northeastern 

Utah: J. Amer. Water Res. Assoc.

Hereford, R., Webb, R.H., and Graham, 

S., 2002, Precipitation history of the 

Colorado Plateau region, 1900-2000: U.S. 

Geol. Surv. Fact Sheet 119-02, 4 p.

Hidalgo, H.G., in press, Climate precursors 

of multidecadal drought variability in the 

western U.S.: Water Resources Res.

Kerr, R.A., 2000, A North Atlantic climate 

pacemaker for the centuries: Science, 

288: 1984-1986.

LaRue, E.C., 1925, Water power and flood 

control of Colorado River below Green 

River, Utah: U.S. Geol. Surv. WaterSupply Paper 556, 176 p.

Mantua, N.J., and Hare, S.R., 2002, The 

Pacific decadal oscillation: J. Ocean. 58: 

35-42.

McCabe, G.J., Palecki, M.A., and Betancourt, 

J.L, 2004, Pacific and Atlantic Ocean 

influences on multidecadal drought 

frequency in the United States: Proc. Nat. 

Acad. Sci. 101: 4136-4141.

Meko, D.M., Stockton, C.W., and Boggess, 

W.R., 1995, The tree-ring record of 

severe sustained drought: Water Res. 

Bull. 31: 789-801.

Schubert, S.D., Suzrez, M.J., Pegion, P.J., 

Koster, R.D., and Bacmeister, J.T., 2004, 

On the cause of the 1930s Dust Bowl: 

Science 303: 1855-1859.

Stewart, I.T., Cayan, D.R., and Dettinger, 

M.D., 2004, Changes in snowmelt runoff 

timing in western North America under 

a ‘business as usual’ climate change 

scenario: Clim. Change 62: 217-232.

Webb, R.H., and Betancourt, J.L., 1992, 

Climatic variability and flood frequency 

of the Santa Cruz River, Pima County, 

Arizona: U.S. Geol. Surv. Water-Supply 

Paper 2379, 40 p.

For further information, contact:

Office of the Chief Scientist for Hydrology

U.S. Geological Survey

436 National Center

Reston, VA 20192

(703) 648-5041

gs-w csh@usgs.gov