information:highlighting
20th and 21st centuries

The "Big Freeze" of 1918–19 blocked river traffic north of Memphis, Tennessee, preventing transportation of coal from southern Illinois. This resulted in widespread shortages, high prices, and rationing of coal in January and February.

In the spring of 1927, the river broke out of its banks in 145 places, during the Great Mississippi Flood of 1927 and inundated 27,000 sq mi (70,000 km) to a depth of up to 30 feet (9.1 m).

In 1930, Fred Newton was the first person to swim the length of the river, from Minneapolis to New Orleans. The journey took 176 days and covered 1,836 miles.

In 1962 and 1963, industrial accidents spilled 3.5 million US gallons (13,000 m) of soybean oil into the Mississippi and Minnesota rivers. The oil covered the Mississippi River from St. Paul to Lake Pepin, creating an ecological disaster and a demand to control water pollution.

On October 20, 1976, the automobile ferry, MV George Prince, was struck by a ship traveling upstream as the ferry attempted to cross from Destrehan, Louisiana, to Luling, Louisiana. Seventy-eight passengers and crew died; only eighteen survived the accident.

In 1988, the water level of the Mississippi fell to 10 feet (3.0 m) below zero on the Memphis gauge. The remains of wooden-hulled water craft were exposed in an area of 4.5 acres (1.8 ha) on the bottom of the Mississippi River at West Memphis, Arkansas. They dated to the late 19th to early 20th centuries. The State of Arkansas, the Arkansas Archeological Survey, and the Arkansas Archeological Society responded with a two-month data recovery effort. The fieldwork received national media attention as good news in the middle of a drought.

The Great Flood of 1993 was another significant flood, primarily affecting the Mississippi above its confluence with the Ohio River at Cairo, Illinois.

Two portions of the Mississippi were designated as American Heritage Rivers in 1997: the lower portion around Louisiana and Tennessee, and the upper portion around Iowa, Illinois, Minnesota, Missouri and Wisconsin. The Nature Conservancy's project called "America's Rivershed Initiative" announced a 'report card' assessment of the entire basin in October 2015 and gave the grade of D+. The assessment noted the aging navigation and flood control infrastructure along with multiple environmental problems.

In 2002, Slovenian long-distance swimmer Martin Strel swam the entire length of the river, from Minnesota to Louisiana, over the course of 68 days. In 2005, the Source to Sea Expedition paddled the Mississippi and Atchafalaya Rivers to benefit the Audubon Society's Upper Mississippi River Campaign.