Western Rivers System
This system of markers is used on the Mississippi River and its tributaries above Baton Rouge, Louisiana, and on some other rivers that flow toward the Gulf of Mexico. The major difference from the U.S. Aids to Navigation System lateral markers shown previously is that navigation markers on the Western Rivers System are not numbered. Numbers displayed below daymarks along this system are not associated with the right or left side of the channel; these numbers indicate the distance from the river's mouth.
However, on the Ohio River, the numbers below the marker indicate the distance from the headwaters. And on the upper Mississippi River, the mileage is measured from Cairo, Illinois at the junction of the Ohio River.