The Mississippi River is the largest North American waterway, draining an immense area that spans 32 states and two Canadian provinces. This vast drainage basin means that heavy rainfall or significant snowmelt anywhere across the central continent can contribute to high water levels downstream. Because the river stretches over 2,300 miles, the term “flood” can mean something vastly different depending on the location and the scale of the event. To understand when the Mississippi River last flooded, it is necessary to first establish what qualifies an event as a major flood.
Defining a Major Flood Event
A major flood event is distinguished from routine seasonal high water by its severity, duration, and the necessary activation of extensive flood control measures. The National Weather Service (NWS) uses a tiered system to categorize river conditions, ranging from Minor to Moderate to Major. Minor flooding involves minimal property damage but may cause public inconvenience and threaten commerce, while Moderate flooding requires some evacuations and road closures.
A Major Flood involves extensive inundation of structures and both primary and secondary roads, typically requiring significant evacuations and the relocation of property. The United States Army Corps of Engineers (USACE) defines a major event by its potential to stress the Mississippi River and Tributaries (MR&T) flood control system. This requires widespread, prolonged high water across multiple states, often necessitating the operation of diversion structures.
The Most Recent Significant Flood
The most recent significant and prolonged flood event on the Mississippi River occurred throughout 2019 and into the spring of 2020. The 2019 event was characterized less by its peak height and more by its unprecedented duration and total volume of water. The initial high water was set up by a series of heavy rainfalls in the fall of 2018, which saturated the ground, followed by a rapid snowmelt and a “bomb cyclone” storm in the spring of 2019 across the Missouri River and Upper Mississippi basins.
This combination of factors led to a historic duration of high water that broke records at multiple gauge stations along the Upper Mississippi River. For instance, in Burlington, Iowa, the river remained above flood stage for more than 104 consecutive days, shattering the previous record of 41 days set during the 1993 flood. The sheer volume of water moving south was also record-breaking; the annual mean discharge recorded at Memphis, Tennessee, in 2019 was the highest in the period of record dating back to 1933. This prolonged inundation led to an estimated $20 billion in total damages across the river basin and prompted the activation of the Bonnet Carré Spillway near New Orleans in two consecutive years (2019 and 2020).
Benchmarks in Mississippi River Flooding History
Historically, three floods serve as the primary benchmarks against which all other Mississippi River events are measured, each providing a turning point in flood management policy. The Great Flood of 1927 remains the most destructive in US history, inundating 27,000 square miles and leading directly to the establishment of the modern USACE flood control system. This event demonstrated the failure of the prior “levees-only” policy, resulting in the Flood Control Act of 1928.
The Great Flood of 1993 was the largest flood disaster in the Upper Mississippi River basin, causing up to $16 billion in damages. This flood stressed the system for months, with some locations remaining above flood stage for over 190 days. While the 1993 flood was catastrophic for the upper and middle river, the 2011 flood represents the modern era’s largest water volume event, testing the limits of the lower river’s control structures.
During the 2011 event, the USACE opened the Bonnet Carré Spillway and, for only the second time in its history, the Morganza Spillway to divert massive amounts of water from the main channel. The controlled diversion successfully lowered the river stage, preventing catastrophic levee failures and protecting the densely populated areas of Baton Rouge and New Orleans.
The Current System of Flood Mitigation
The current strategy for managing the Mississippi River’s flow is the Mississippi River and Tributaries Project (MR&T), an extensive public works system designed and operated by the USACE. This system relies on a combination of structures to contain the “project flood,” a hypothetical event larger than the 1927 flood. The first component is the mainline levee system, a vast network of earthen embankments and concrete floodwalls that spans over 2,200 miles along the river and its major tributaries.
The second component involves control structures and spillways, which are engineered gates designed to divert excess water away from the main river channel. The Bonnet Carré Spillway, located upstream of New Orleans, is one such structure that redirects water into Lake Pontchartrain, lowering the river stage protecting the city. Similarly, the Morganza Spillway, when opened, diverts water into the Atchafalaya River Basin, an engineered floodway.
These floodways, which include the Atchafalaya Floodway, form the third and final line of defense, providing a designed path for floodwaters to safely bypass critical population centers. The system’s operation is dictated by complex protocols to minimize risk to the largest number of people and the most valuable infrastructure.