After the Mississippi River passes St. Louis it begins to change character. The river north of St. Louis is punctuated with locks and dams that allow river boat traffic to navigate the steep slope that the river follows. South of St. Louis the slope becomes gentler. At Cape Girardeau, Missouri the river passes the northernmost point of the Crowley's Ridge. Crowley's Ridge delineates the western edge of the Mississippi River Valley through southeastern Missouri and western Arkansas. The Mississippi River Valley at this point is called the Mississippi Embayment and the Crowley's Ridge can be as far as 150 miles west from the current river channel. When the Mississippi River meets the Ohio River at Cairo, Illinois it is halfway on its journey to the sea. It is here that the brown muddy water of the Mississippi begins to mingle with the clearer water of the Ohio. On a sunny day you can see the difference from Fort Defiance Park in Cairo or from Fort Jefferson Hill just south of Wickliffe, Kentucky. Without the locks and dams the Mississippi begins to wind and curve so much so that the distance by water from Cape Girardeau to the Gulf of Mexico is twice the distance as a crow flies. It is because of this meandering flow that it is here that the Mississippi begins to take on the moniker of “Old Man River.”