Mark Twain Lake is a reservoir located in Ralls and Monroe Counties in the U.S. state of Missouri. It was created by the Clarence Cannon Dam impounding the Salt River and is located about 20 miles southwest of Hannibal. The lake was named for Missouri author Mark Twain and part of the area around it is Mark Twain State Park. The village of Florida, th…Mark Twain Lake is a reservoir located in Ralls and Monroe Counties in the U.S. state of Missouri. It was created by the Clarence Cannon Dam impounding the Salt River and is located about 20 miles southwest of Hannibal. The lake was named for Missouri author Mark Twain and part of the area around it is Mark Twain State Park. The village of Florida, the birthplace of Mark Twain, is mostly surrounded by the lake.