Mississippi River in the Magic Kingdom
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As you walk from Liberty Square, into Frontierland, just before the Diamond Horseshoe, there is a metal plate that goes across the entire width of the walkway. Under that plate, water runs to the lake.
Google Map view of the Mississippi Bridge.
This stream represents the Mississippi River. To the east is Liberty Square symbolizing the eastern part of the United States with its colonial architecture and early American theme.
To the west is the early American frontier of Frontierland. If you look at the pavement, it changes on either side of the "Mississippi" following the Magic Kingdom's design of changing the walkways as you move from one land to another.