Rivers naturally rise and fall, a natural cycle that is necessary to sustain the variety of life in its ecosystem. Some plants need high water to do well, while others won’t grow until some of the river dries out. Different kinds of wildlife rely on these plants for their diet and well-being. Periods of high water also replenish ground water, as long as the water is allowed to naturally seep in and isn’t funneled away by concrete roads and drainage tunnels. In most years, river levels peak in spring with runoff from melting snow and early spring rains, but rises in June and October are also common.

Our efforts to control the river by forcing it into a narrow channel and raising levees and walls along its banks have contributed to higher river levels than existed before our interventions. In addition, water levels in the Mississippi and its tributaries are higher because of increased development in the watershed (for example, paving over the landscape), draining wetlands, and forcing runoff into concrete-lined drainage ditches.

The impact of development on river levels has been understood since at least 1852 when Charles Ellet wrote a report well ahead of its time. He argued that higher water levels on the lower river were caused by: 1) increased farming in the floodplain; 2) the construction of levees; 3) building cutoffs that straighten the river and increase the surface speed of the water; and 4) the continued deposits at the delta that were extending the land mass further into the sea, decreasing the slope of the river and causing more backup into the delta region.

So we shouldn’t be surprised that flooding along the Mississippi is becoming more common and that we’re seeing higher water levels than we’ve seen before. These conditions continually threaten our existing levees and flood barriers, but the cost of raising them over and over again is prohibitive and doesn’t solve the core problem. We need to re-exmaine how we develop in floodplains and how we manage water.

As noted earlier, flooding—waters levels that rise above the river’s normal embankment—are common. Since we started living along the river and taking note of its mood swings, we have noted major flooding in: 1785, 1844, 1849, 1851, 1859, 1862, 1870, 1880, 1882, 1888, 1892, 1881 (fall), 1912, 1916, 1920, 1922, 1927, 1937, 1938, 1942, 1951, 1952, 1965, 1969, 1973, 1996, 1997, 2001, 2008, & 2011.

Folks in the media like to throw around the phrase “100-year flood” during major floods because it makes for a nice sound bite, but the term is widely misunderstood. It does not mean the type of flood that one would expect to happen only once a century. It just means there is only a 1% chance that the river would reach that level of flooding in any given year. There is the same 1% chance of that happening every year. Incidentally, the records we use to calculate those odds are based on barely more than a century’s worth of data, so today’s 100-year flood could be tomorrow’s 25-year flood as more data are collected.

There are common misconceptions about how flooding in one part of the river impacts the rest of the river. The reality is that flooding along the Upper Mississippi River has little impact on river levels on the Lower Mississippi. The carrying capacity of the river increases dramatically after the confluences with the Missouri and Ohio Rivers, so high water from the Upper River—even record levels—can be absorbed with little impact south of Cairo, Illinois. Record floods along the Upper Mississippi River in 1993 did not push the Lower Mississippi River above flood stage because the Ohio River—which contributes half of the volume of the Mississippi River south of Cairo—was behaving itself.

Here’s a summary of the really big floods that have happened along the Mississippi since we began building homes near it. As a side note, the river levels mentioned below are not based on the actual depth of the river but on the historically low river levels observed in 1864. In most places, a river level of 0 does not mean no water is in the river, but rather it means a river level equal to the 1864 gauge at that spot.

The year of the greatest flood along the Upper Mississippi River, at least in terms of water volume, it had minimal economic impact because the number of people living along the river at that time was small. The winter of 1843-44 was bitterly cold with heavy snows. Unusually cold weather lingered into spring, then heavy rains arrived. By early May, the river had spread across the floodplain, stretching 2 ½ miles at St. Louis. By June 7, the river was back in its banks, but this proved to be a brief reprieve. The next day, the river began to rise again on the tails of more heavy rain and high water coming down the Missouri River. The river at St. Louis peaked again on June 27 at just over 41 feet, with an estimated flow of 1,000,000 to 1,300,000 cubic feet per second.

Heavy rains early in the year brought high water along the Lower Mississippi from January to March. At New Orleans, the river was above flood stage for 91 days. Levees, most of them built by local and state governments, were breached in many places from Memphis south. Water spread nearly 20 feet wide in places, inundating homes and washing out rail lines. The attic (or roof) became the primary living space for folks in flooded areas. This flood triggered the first major effort from the federal government to coordinate and build levees along the Lower River.

Persistent heavy rain in the southern basin (from the Illinois River south) that began in the winter of 1926 pushed the Mississippi to record levels which peaked in April and May, creating one of the worst disasters in modern history. The Mississippi River overwhelmed levees, covering 27,000 square miles of the flat, wide flood plain. The river stretched up to 70 miles wide and was up to 30 feet deep. The flood waters displaced 900,000 people; hundreds died.

The flood led to a major change in policy for flood management. Previous to 1927, the US Army Corps of Engineers had pursued a flood control policy that relied heavily on levees at the expense of everything else. In 1928, Congress passed the Flood Control Act and the Corps was forced to use a more varied approach to flood control, including levees, floodways, outlet channels, channel stabilization, and tributary basins. This was quite possibly the largest flood control project in the world, at least at that time.

Just ten years after the big one, heavy rains in the Ohio River basin again pushed the Ohio and lower Mississippi Rivers to near-record levels. This time around unusually heavy rain in January raced off the frozen ground and into the rivers.

Watch footage of the high water at Cairo, Illinois, a city at the confluence of the two rivers, here.

The most destructive flood along the Upper Mississippi north of the Quad Cities was in April of 1965. Almost every river town recorded their highest water levels in that year. The early part of 1965 was unusually wet and cold. Ice on the river formed dams that, when they finally broke, sent torrents of water cascading downriver. Thousands of volunteers piled sandbags on top of makeshift levees to try to contain the Mississippi. At Winona, the river peaked on April 20 at a record 20.75 feet (flood stage was 12 feet) but the sandbag wall—up to 20 feet tall and nine miles long—held, in part because of a levee break downriver had relieved some pressure on the Winona wall. The crest reached La Crosse on April 21, reaching a record 17.9 feet (flood stage is 12 feet). About 200 homes were flooded. Downriver at Prairie du Chien, the impact was more significant. A quarter of the city’s homes were evacuated as the river peaked at a record 25.57 feet (seven feet above flood stage).

The final tally was 19 dead, 40,000 people displaced from their homes, and damages estimated at $200 million. This “100-year flood” was followed by another “100-year flood” just four years later.

This was the record-breaking flood for communities along the central Mississippi River from the Quad Cities to Cairo, Illinois (as well as for many communities along the Missouri River). The primary cause was unusually heavy, persistent rain in the Upper Midwest; already saturated soils could not absorb anymore water, so the water ran directly to streams and rivers. At St. Louis, the river was above flood stage from April 1 to September 30—a total of 144 days. The river level peaked on August 1 at 49.58 feet or just 2½ feet below the top of the city’s flood walls; flood stage is 30 feet. The Missouri River also reached record crests around the same time. High water wiped out 80% of levees along the Mississippi, flooding 17,000 square miles flooded. 74,000 people were displaced, and 50,000 homes destroyed, but only 52 deaths were attributed to the flood.

In the aftermath, there was much public debate about the causes, including the relative contribution of development along the river and human interventions to control the river for navigation. It is worth noting that, although the flood set records for the height of the flood waters, the estimated volume was still lower than the 1844 flood, which crested at St. Louis about 8 feet lower than the 1993 flood. For a few years immediately following the flood, efforts were made to move communities to higher ground—the small town of Valmeyer, Illinois relocated to the tops of nearby bluffs—but some communities insisted on building new, taller levees instead. In the floodplain of the Missouri River at Chesterfield, Missouri, areas that were inundated with over 10 feet of water are now home to new, post-flood strip malls and shopping centers.

Many communities along the Upper Mississippi experienced major flooding, with peak river levels second only to the 1965 flood, but damage to communities is minimal.

Some communities around the Des Moines River confluence had Mississippi River levels that surpassed the 1993 records, although water levels did not stay high nearly as long as they did in 1993.

Check out pictures from flooding in 2008 here.

Another big year for flooding, especially along the lower Mississippi River. Early in the year, it seemed likely that the upper river was going to be hit hard. River levels stayed unusually high through the winter after a wet fall. In addition, the northern sections of the watershed had a lot of snow and much of it was still on the ground in early March. However, the snowpack melted gradually and spring rains weren’t especially heavy. Although water levels stayed high for weeks, flooding along the upper river was minor. It was a very different story for the lower river, however.

The Ohio River watershed also had a deep snowpack. Relentless heavy rain in the spring along the Ohio and Missouri River basins dumped torrents of water into the Mississippi. The river gage at Cairo, Illinois, reached a record high on May 2 at 61.72 feet.

On May 3, the US Army Corps of Engineers blasted a hole in the Bird’s Point Levee (after a court challenge trying to stop the blasting failed), sending water pouring into the 210-square mile Birds Point-New Madrid Floodway, an area long designated for overflow but never intentionally used as such. (High water topped the levee in 1937 and flooded the area but it was not an intentional breach). Some 200 residents were flooded out and 130,000 acres of land was submerged, most of it farmland.

The high water rolled down river, cresting at Memphis on May 10 just one foot shy of the record; some tributaries backed up and flooded Memphis neighborhoods. The casinos in Tunica County closed for several weeks; many of them got wet. At Vicksburg, the river crested on May 18 at a new record high of 57.17 feet; it didn’t fall below flood stage (43 feet) until June 16.

On May 14, the Corps opened one gate of the Morganza Spillway to protect Baton Rouge and New Orleans; eight more gates were opened the next day. The spillway hadn’t been used since 1973. That still wasn’t enough, though. On May 23, the Corps opened the gates to the Bonnet Carre Spillway, diverting water to Lake Pontchartrain and the Gulf; the gates weren’t closed until June 20.

The Corps’ flood protection system essentially worked as intended, but flood damages were still estimated at $5 billion to property and crops. And, the Corps will be spending hundreds of millions of dollars to rebuild the flood control system that was damaged in 2011.

Footnote: May 26, 2011. I’m in Grand Rapids, Minnesota, in the Headwaters region, and the river is high enough to flood a few low-lying roads. In St. Louis, the river is expected to peak a few feet above flood stage again this weekend. Meanwhile, we all know what’s going on with the record flooding along the Lower Mississippi. What’s remarkable to me is that each of these sections is really a different river; flooding on one stretch has only minimal impact on the water levels in other stretches. Yet, here we are in spring 2011 with flooding along these very different parts of the Mississippi River. I don’t know what that means, but it sure is interesting.

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© Dean Klinkenberg, 2011