The blue on the map, generated from satellite data used to detect flooding, shows the estimated extent and intensity of flooding . The usual extents of rivers are shown in white. Farmland , much of which was so drenched that farmers had to delay planting, is shown in green.

This year’s flooding across the Midwest and the South affected nearly 14 million people, yet the full scale of the slowly unfolding disaster has been difficult to fathom. To visualize just how extensive it was, The New York Times created this composite map showing all the areas that were inundated at some point from January through June.

Public interest in natural disasters tends to focus on big, discrete weather events like hurricanes. But flooding that unfolds over months across a broad area has a harder time breaking through. It is only when seen as a single, connected event that the stunning scale of the 2019 flood season becomes clear.

To measure the scope of the spring floods, The New York Times analyzed satellite data from the Joint Polar Satellite System using software, developed by government and academic researchers for flood detection, that is frequently used in disaster response.

The data covers the period from Jan. 15 to June 30 and shows an interconnected catastrophe along the Missouri, Mississippi and Arkansas Rivers, a system that drains more than 40 percent of the landmass of the continental United States.

Areas that flooded NORTH DAKOTA Missouri River MINNESOTA WISCONSIN SOUTH DAKOTA IOWA NEBRASKA ILLINOIS Arkansas River KANSAS MISSOURI TENNESSEE ARKANSAS OKLAHOMA Mississippi River LOUISIANA TEXAS MISSISSIPPI N.D. Missouri River MINN. WISC. S.D. IA. NEB. ILL. Arkansas River KS. MO. TENN. ARK. OKLA. Mississippi River LA. TEX. MISS. The New York Times

The flooding started in earnest in March as heavy rains fell on frozen ground already bearing a deep load of winter snow. As the water spread across parts of South Dakota, Nebraska and Iowa, traditional defenses that communities had in place to protect against rising waters were quickly overwhelmed.

As the high water from the many tributaries of the Mississippi came together, the river became a record-breaking monster.

Major flood stage sites, Jan. 15 to June 30 Federal assistance-approved counties NORTH DAKOTA Missouri River MINNESOTA WISCONSIN SOUTH DAKOTA IOWA NEBRASKA ILLINOIS Arkansas River KANSAS MISSOURI TENNESSEE ARKANSAS OKLAHOMA Mississippi River LOUISIANA TEXAS MISSISSIPPI N.D. Missouri River MINN. WISC. S.D. IA. NEB. ILL. Arkansas River KS. MO. TENN. ARK. OKLA. Mississippi River LA. TEX. MISS. Source: United States Geological Survey | The New York Times

By the end of June, the flooding was so intense and widespread that at least 11 states had sought federal disaster funds for more than 400 counties. Forty-nine United States Geological Survey gauges measured more water this year than at any time in at least 20 years.

Bryan Tuma, assistant director of Nebraska’s Emergency Management Agency, said, simply, “I would describe it as biblical.”

Floodwaters spread across the town of Hamburg, Iowa, in March. Tim Gruber for The New York Times

The causes of flooding are complicated, but climate change is increasingly an exacerbating factor. Warmer air can hold more moisture, and that moisture can fall back out of the sky, whether as rain or snow, in greater amounts.

The year through May 2019 was the wettest 12-month period on record in the United States, according to the National Oceanic and Atmospheric Administration. Nearly 38 inches of water fell, almost eight inches above average.

Flooding outside of Fortescue, Mo., damaged the levee designed to hold back water. Tim Gruber for The New York Times

A Mississippi River mayors council estimated that the cost of infrastructure damage and emergency response was at least $2 billion. That number is likely to rise as the water recedes and officials can check the extent of the damage. The full cost to repair homes and businesses has yet to be calculated.

David Alexander, a professor of risk and disaster reduction at University College London, said that typical recovery times from major disasters are “in the range of 10 to 25 years.”

A boat made its way through a flooded street in Grafton, Ill., in May. Whitney Curtis for The New York Times

Ultimately, the volume of water is only one source of damage: What’s in that water also plays a role.

The waters of the Mississippi, carrying chemical fertilizers from heartland farms, lawns and other sources, have contributed to a “dead zone” in the Gulf of Mexico, an area with too little oxygen to support fish and other marine life.

NOAA predicts that this year's dead zone will be 8,717 square miles — about the size of New Hampshire.

Farms in the lower Mississippi Delta remained underwater in June. Rory Doyle for The New York Times

Back in March, Edward Clark, director of NOAA’s National Water Center, said that what lay ahead was a “potentially unprecedented” flood season, even worse than the Great Flood of 1993.

In a recent interview, Mr. Clark said, with no satisfaction, “Yes, we got it right.”

Sixty-three percent of comparable United States Geological Survey gauges in the region recorded higher peaks this year than in the same period of 1993.

“This is a year that will remain in our cultural memory, in our history,” he said.