From its headwaters at Lake Itasca in N. Minnesota the Mississippi river drains 41% of the contiguous United States and 15% of North America, before emptying into the Gulf of Mexico.
Approximately 90 days is the retention time or travel time of water from the headwaters to the G. of Mexico. The Mississippi drainage basin is over 3.2 million km2 (or 1.2 million mi2) and is a source of water to roughly 18 million people as well as industry and agriculture.
Much of the catchment area is intensively cultivated producing 92% of the US’s agricultural export and 60% of all grain exported from the US is transported via the river. To maintain water way traffic and control flooding the river has been heavily modified by engineering works creating 43 dams in the upper Mississippi and thousands of wing dams throughout the river system. This levee system has changed the flow, geography and ecology of the river and its watershed area.