River miles are distance measurements by the U.S. Army Corps of Engineers that measure miles along a navigable waterway. The mile numbers begin with zero (at the mouth) and increase with each mile you travel upstream. You use river miles to locate places along the river, relative to the location of the mouth. While most rivers in the United States already are marked, you can mark river miles along smaller channels, too, using simple tools and calculations.
Things You'll Need
 Topographic map or navigation chart
 Map wheel or a length of string and a ruler
 Pencil or pen and a piece of paper
 Calculator
Using a Map Wheel to Calculate River Miles

Find the length of the waterway. Place the map wheel on the map at the mouth of the waterway. Move the wheel slowly up the center of the waterway to the location you wish to measure, and note the distance traveled in inches.

Calculate the distance. Topographic maps use a 1:24,000 scale, where 1 unit on the map equals 24,000 of the same unit on the ground. Each inch therefore represents 0.3788 miles. Divide the total number of inches, including fractions, by 0.3788 and the result will be the number of river miles on the measured waterway.

Mark your map with the river mile marks at 2.64 inch intervals, starting with zero at the mouth.
Using a String and Ruler to Calculate River Miles

Find the length of the waterway. Position the string carefully over the center of the waterway mouth, and then lay the string down along the length of the waterway to the location you wish to measure. Mark the string at the start and end points. Straighten the string and use the ruler to measure it (inches).

Calculate the distance. Topographic maps use a 1:24,000 scale, where 1 unit on the map equals 24,000 of the same unit on the ground. Therefore, 1 inch represents 0.3788 miles. Divide the total number of inches, including fractions, by 0.3788 and the result will be the number of river miles on the measured waterway.

Mark your map with the river mile marks at 2.64 inch intervals, starting with zero at the mouth.