The Mississippi River has the world's fourth largest drainage basin ("watershed" or "catchment"). The basin covers more than 1 245 000 sqmi including all or parts of 31 U.S. states and two Canadian provinces. The drainage basin empties into the Gulf of Mexico, part of the Atlantic Ocean. The total catchment of the Mississippi River covers nearly 40% of the landmass of the continental United States. The highest point within the watershed is also the highest point of the Rocky Mountains, Mount Elbert at 14 440 ft.
In the United States, the Mississippi River drains the majority of the area between crest of the Rocky Mountains and the crest of the Appalachian Mountains, except for various regions drained to Hudson Bay by the Red River of the North; to the Atlantic Ocean by the Great Lakes and the Saint Lawrence River; and to the Gulf of Mexico by the Rio Grande, the Alabama and Tombigbee rivers, the Chattahoochee and Appalachicola rivers, and various smaller coastal waterways along the Gulf.
The Mississippi River empties into the Gulf of Mexico about 100 mi downstream from New Orleans. Measurements of the length of the Mississippi from Lake Itasca to the Gulf of Mexico vary somewhat, but the United States Geological Survey's number is 2320 mi. The retention time from Lake Itasca to the Gulf is typically about 90 days.