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