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by Bill Frederick

Senior National Weather Service Meteorologist

& NWS Liaison at the Mississippi Valley Division

March 2019
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looding along the Mississippi River has increased both in frequency and magnitude 

during the past three to five decades. During the 49-year period of 1970 to 2018, 

stages at Cairo, Illinois - the confluence of the Mississippi and Ohio rivers - have 

exceeded major flood stage of 52.0 feet 16 times, as opposed to 13 times during the 72-

year period of 1898 to1969. 

The 2002 U.S. Department of State Climate Action Report stated that total annual 

precipitation has been increasing, not only across the United States but worldwide, over 

the past 100 years. In his 2009 paper, Global Climate Change Impacts in the United 

States, Thomas R. Karl concludes that total annual precipitation for the contiguous United 

States has increased an average of about 5 percent over the past 50 years when 

compared to the 1901-2012 average. Walsh et al. (2014) quantifies the increase as 4 

percent nationally during the period of 1901 to 2015. Basically, precipitation is increasing 

nationally (Figure 1).

The Midwestern and New England states have shown the greatest increase in annual 

precipitation during the past 50 years (Figure 1). When dealing with flooding in the 

Mississippi Valley, annual increases may not be the best benchmark since the majority of 

flooding occurs during the spring. 

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FIGURE 1. Increase in annual precipitation for the period 1966-2015 as compared to the 1901-2015 average.
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lthough winter precipitation is 

about the same over the Mississippi 

watershed, spring precipitation is 

markedly higher over the Ohio, Missouri and 

Mississippi watersheds, upstream from the 

confluence of the Mississippi and Ohio rivers 

(Figure 2). The greatest increases in 

seasonal precipitation have occurred over 

the Arkansas/Red/lower Mississippi valleys 

and over Minnesota and the Missouri Valley 

during the fall (Figure 2). It is vitally 

important to recognize that the northern, 

increased precipitation during the fall 

generally reflects the moister soil going into 

the winter freeze, allowing for more water 

to be available for spring melt. 

Local studies have quantified the 

percentages by basin over the Mississippi 

watershed. 

FIGURE 2. Seasonal changes in precipitation for the period 1966-2015 as compared with the 1901-2015 averages.

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2015-2016 winter flood: Railroad bridge 

under water in Valley Park, Missouri 

Valley Park, Missouri, during 2015-2016 winter flood
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able 1 reflects the 

percentage of increase in 

precipitation during the 30

-year period of 1982-2011, as 

compared with the 1901-2011 

averages over the Mississippi 

watershed’s various basins. Data, 

compiled from the National 

Climate Data Center in Asheville, 

North Carolina, reflects each 

basins weighted average of all 

climate divisions upstream from 

the basin. 

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BASIN DEC-FEB MAR-MAY JUN-AUG SEP-NOV ANNUAL

Upper Mississippi River 

at Grafton, IL +6.9 +6.3 +5.4 +8.2 +6.6

Missouri River at 

Hermann, MO -1.1 +6.6 +1.8 +7.8 +4.2

Mississippi River at 

St. Louis, MO +2.0 +6.5 +3.0 +8.0 +5.0

Ohio River at 

Metropolis, OH +0.2 +3.8 +2.2 +11.4 +4.3

Cairo, IL (confluence of 

Miss. & Ohio rivers) +0.9 +5.0 +1.8 +8.7 +4.2

Arkansas River at 

Little Rock, AR +11.3 +4.4 +2.5 +8.3 +5.8

Red River at 

Alexandria, LA +5.9 -3.8 +3.6 +9.7 +3.7

Entire Mississippi River 

Watershed +3.1 +3.0 +2.0 +9.2 +4.3

TABLE 1. Seasonal changes in precipitation for 1982-2011, as compared to 1902-2011 over the various basins that 

comprise the Mississippi watershed.

2011 flood: 

A community 

protected by a 

Mississippi 

River levee
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eighted averages were calculated by basin for the 30-year running average 

over the watershed upstream from the confluence of the Mississippi and Ohio 

rivers at Cairo, Illinois (Figure 3).

It is noted that annual precipitation values have steadily increased since the 1970s. 

The difference is 7.8 percent when comparing the 1982-2011 period with the 1932-1961 

timeframe, and 8.4 percent when comparing the 26-year average from 1992-2017 with 

the same 1932-1961 period. Increases in seasonal and annual precipitation over the 

Mississippi watershed are not the only reason for the increase in flooding that has 

occurred during the past several decades. Heavy rains from extreme rainfall events also 

increased dramatically during the 1992-2017 time period. 

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FIGURE 3. 30-year running weighted average of precipitation over the watershed upstream from Cairo, Illinois.

2013 flooding on the upper Mississippi River
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ccording to the 2014 National 

Climate Assessment, “Heavy 

downpours are increasing 

nationally, especially over the last three 

to five decades. The heaviest rainfall 

events have become heavier and more 

frequent, and the amount of rain falling 

on the heaviest rain days has also 

increased. Since 1991, the amount of 

rain falling in very heavy precipitation 

events has been significantly above 

average. This increase has been 

greatest in the Northeast, Midwest and 

Upper Plains – more than 30 percent 

above the 1901-1960 average. There 

has also been an increase in flooding 

events in the Midwest and Northeast, where the largest increases in heavy rain amounts 

have occurred.” Figure 4 denotes decadal trends in heavy precipitation during 

the 112-year period from 1900 to 2012.

FIGURE 4. Kunkel, K. E. et al., 2013: Monitoring and understanding trends in extreme storms: State of knowledge. 

Bulletin of the American Meteorological Society, 94, doi:10.1175/BAMS-D-11-00262.1.

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2016 flooding in Cedar Rapids, Iowa
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ccording to the article 

Monitoring and Understanding 

Trends in Extreme Storms

(Kunkel, et al 2013), “There is strong 

evidence for a nationally averaged 

upward trend in the frequency and 

intensity of extreme precipitation 

events. The COOP (Cooperative 

Observer) network is considered 

adequate to detect such trends. The 

causes of the observed trends have not 

been determined with certainty, 

although there is evidence that 

increasing atmospheric water vapor 

may be one factor.” Figure 5 shows 

a significant increase in two-day rainfall 

events that have exceeded the five-year 

recurrence interval over the contiguous 

U.S. since the 1990s.

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FIGURE 5. Index of the number of two-day precipitation events exceeding the station-specific threshold for a five-year 

occurrence interval in the contiguous U.S.

2011 flood: Raising needles at the Bonnet Carré Spillway
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arl et al. (2009) breaks down the percentages of increase in precipitation that has 

fallen in very heavy rain events from 1958 to 2012 by region as compared to the 

100-year average (Figure 6). A marked 37 percent increase in rain that has fallen K

during very heavy rain events is noted over the Midwestern states.

FIGURE 6. Percentage of increase in the amount of precipitation falling in very heavy rain events by region.

2015-2016 flood: Flooding in the St. Louis 

District’s area of operation

Flood water and debris at Lock and Dam 20 on the 

Mississippi River in Canton, Missouri, 2014
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o, how has the Mississippi River responded to the increase in precipitation? 

Figure 7 depicts the percentage of years that Cairo (at the confluence 

of the Ohio and Mississippi rivers) has exceeded the National Weather Service’s 

(NWS) minor/moderate/major flood stage per 30-year period. It is noted that there is 

minimal difference in the percentage (83-97 percent) of minor flood stage occurrences 

during the 30-year periods. 

When comparing moderate flood stages, after a 47 percent and 50 percent probability 

during the first two periods, the remainder of the periods were also fairly uniform ranging 

from 60-73 percent. The 

greatest percentage difference has occurred in the 

major flood stage category. 

While the first seven 30-year 

periods ranged from 13-20-

percent occurrences, the last 

three 30-year periods have 

risen to about double that, 

with 30-38 percent of the 

years from 1972 to 2017 

exceeding the NWS major 

flood stage of 53 feet at 

Cairo. 

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FIGURE 7. Percentage of years exceeding the NWS minor/moderate/major flood stages per 30-year period at Cairo.

2011 flood: Near the Birds Point-New Madrid Floodway, in southeast 

Missouri just below the confluence of the Ohio and Mississippi rivers
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his rise in percentages coincides with both 

the increases in precipitation trends and the 

frequency of very heavy rain events 

discussed earlier. It should also be noted that the 30-

year average of maximum annual stages (represented 

by the yellow line in Figure 7) has increased from 

approximately minor flood stage during the 1952-1981 

time period to above moderate flood stage during the 

1992-2017 period. 

Proceeding farther downstream to Arkansas City, 

Arkansas, at the confluence of the Arkansas and 

Mississippi rivers, an interesting trend occurs. 

Although there is a corresponding increase in 

precipitation (Figure 8) and very heavy rain events as 

at Cairo, flood occurrences have diminished 

dramatically since the 1940s (Figure 9). This 

corresponds with the river cut-off program that began 

in the 1940s which was completed in the 1960s along 

the Mississippi River from Memphis, Tennessee, to 

near Natchez, Mississippi. Although there was a 

significant decrease in the past, recent changes are 

showing aggradation and slightly higher stages in this 

area.

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FIGURE 8. 30-year running weighted average precipitation for the Mississippi watershed at Arkansas City, Arkansas.
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Figure 9. Percentage of years exceeding the National Weather Service’s minor/moderate/major flood stage per 30-year 

period at Arkansas City, Arkansas.

FIGURE 10. 30 year running weighted average precipitation at Red River Landing, Louisiana.

ontinuing downstream to Red River Landing, Louisiana, at the confluence of the 

Mississippi River and Red River, the steady increase in precipitation is noted once Cagain (Figure 10). 
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FIGURE 11. Percentage of years exceeding NWS minor/moderate/major flood stages at Red River Landing, Louisiana

Operation of Morganza Floodway during the 2011 flood

similar trend to that at Cairo, Illinois, is noted at Red River Landing, with an 

increase in 30-year average moderate flood stages during the last three periods. It 

should be noted that the NWS major flood stage of 64 feet has never been 

exceeded at Red River Landing due to the construction and operation of the Morganza 

Floodway (Figure 11).

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FIGURE 12. Percentage of years when minimum stages occurred at Cairo, Illinois.

ne final note, since maintaining a 9-foot navigation channel is a prime function 

of the Mississippi River and Tributaries (MR&T) project, Figure 12 denotes the 

changes in percentages of the lowest level of the year dropping below 10 feet, 

8 feet and 6.5 feet. The 6.5-feet level is the critical navigation stage at Cairo, Illinois. 

It is noted that since the 1942-1971 time period, the percentage of years when the 

stage failed to exceed the critical navigation stage has dropped steadily. Prior to 1942, 

27-43 percent of the years dropped below 6.5 feet; and after 1942, 3-10 percent of the 

years dropped below 6.5 feet, which coincides with the completion of the reservoirs 

upstream from Cairo, Illinois. 

In conclusion, an increase in 

annual precipitation and a marked 

increase in spring precipitation have 

occurred during the past three to five 

decades. Also, there has been a 37 

percent increase in precipitation 

falling in very heavy rain events over 

the Midwest during the past few 

decades. Therefore, the frequency 

and the magnitude of floods have 

increased due to more rain and a 

significant increase in the frequency 

of very heavy rainfall events.

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2011 flood: Sand boil at Cairo, Illinois
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2019 Regional Flooding

ccording to NOAA’s National Centers for Environmental Information, much above 

average precipitation continued to be experienced into 2018/2019. The latest 12-

month period from March 2018 through Feb. 2019 ranked as the wettest in the 

124-year period of record over the upper Midwest, and second wettest over the Ohio, 

middle Mississippi and lower Missouri valleys. In fact, Sept. 2018 through Feb. 2019 

ranked as the wettest six-month period on record over the entire Mississippi watershed, 

with the exception of the upper Missouri Valley, which experienced only above-average 

precipitation.

FIGURE 13: Regional rakings of precipitation during the periods 

March 2018-Feb. 2019 (top) and Sept. 2018-Feb. 2019 (bottom).

Drone aerial near 

Satartia, Mississippi, 

in the Yazoo Basin

Joshua Beam, Memphis District , 

surveying top of the Dyer County 

Little Levee in 19-degree weather

Opening of Bonnet Carré Spillway, day one

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Bibliography

Karl, T.R. J. M. Melillo, and T. C. Peterson, Eds., 2009: Global Climate Change Impacts in 

the United States. Cambridge University Press, 189 pp.

NOAA National Centers for Environmental Information (NCEI).

Peterson, T. C. et al., 2013: Monitoring and understanding changes in heat waves, cold 

waves, floods and droughts in the United States: State of knowledge. Bulletin of the American Meteorological Society, 94, 821–834, doi:10.1175/BAMS-D-12-00066.1.

Walsh J. et al., 2014 Ch. 2 Our Changing Climate. J.M. Mellow, T (T.C.) Richmond, and 

G.W. Yohe, Eds., U.S. Global Change Research Program, 19-67.
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U.S. Army Corps of Engineers

Mississippi Valley Division

1400 Walnut Street

Vicksburg, MS 39180