If you look closely at the map above, you’ll see the finger of Maryland that reaches over the top of West Virginia. Route 50 passes through that finger, and on Backbone Mountain in Maryland it enters the eastern edge of the Mississippi River watershed.
Beginning at that point, the shape of the land forces the water in all the creeks, streams, and rivers you pass on Route 50 to eventually flow into the Mississippi, if not prevented from doing so by a local land formation, evaporation, consumption by animals or plants, or acts of man. The watershed covers 1.15 million square miles and includes all or parts of 31 states and two provinces in Canada. Route 50 reaches the western end of the watershed at Monarch Crest, Colorado, on the Continental Divide. Some 1,660 of Route 50’s 3,000-plus miles are in the Mississippi River watershed.
But take another look at the map. You’ll see that the Missouri River flows into the Mississippi (at St. Louis, Missouri). Rivers that reach an ocean are generally named for the longest upriver branch. Based on that approach, it should be the Missouri River that flows south of St. Louis to the Gulf of Mexico, not the Mississippi, and we should be entering the Missouri River watershed.
Perhaps the naming difference occurred because it was in 1541, near what is now Natchez in the state of Mississippi, that Hernando de Soto became the first European to report seeing the Mississippi. It wasn’t until 132 years later, in 1673, that Louis Jolliet and Jacques Marquette, French explorers, reported seeing the Missouri.
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The map of the Mississippi River watershed is from the U.S. National Park Service and available for public use.