River Mile System of Ohio

The river mile system is a method to reference locations on streams and rivers of Ohio. Discrete locations on a stream or river are referenced by river miles. River miles are a linear measure that begin at the mouth of a drainage, and increase in an upstream direction along its path.

History of the River Mile System

River mile measurements were hand-drawn on 7.5-minute USGS topographic maps between 1972 and 1975. Since the completion of the index, river mile measurements have been incorporated into the programs and activities of the Division of Surface Water. Many stream features, such as the location of wastewater discharges and water-quality assessment sites, are referenced using the river mile system. The paper river mile maps were scanned in 2006. The scanned images were saved in JPEG image format, and georeferenced using industry-standard GIS software.

Referencing Stream Locations

There are two methods to reference a discrete location using the river mile system. Using the first method, a location can be referenced by naming the stream on which it is exists, then stating the river mile at which it is found. For example, "Scioto River at river mile 5.0" references a point five miles upstream of the mouth of the Scioto River. This method is descriptive and easy to understand; however, the reference is not always unique. Many streams in Ohio have the same name.

The second method to reference a discrete location is to use a river mile string. A river mile string describes the drainage-network pathway to the location. Each network junction is sequentially listed in the river mile string. Network junctions exist at the confluence of drainages. The downstream-most network junction is listed first. The river mile measurement of the location-of-interest is listed last.

The following example explains how to determine the river mile string for the Scioto River at river mile 5.0. The Scioto River is tributary to the Ohio River, which is tributary to the Mississippi River. The Mississippi River drains into the Gulf of Mexico. The river mile string is established by tracing this path back upstream, and recording the linear measurement along each segment. The confluence of the Mississippi and Ohio Rivers is 953.80 miles up the Mississippi River from the Gulf of Mexico. The confluence of the Ohio and Scioto Rivers is 624.93 miles up the Ohio River. The location-of-interest is five-miles up the Scioto River. The river mile string of the location-of-interest is therefore 953.80 624.93 5.0.

Streams and rivers of Ohio drain to Lake Erie or the Ohio River. Therefore, river mile strings begin with a measurement along the Lake Erie coastline, or along the Ohio River. Measurements along the Lake Erie coastline begin at the mouth of the Saint Lawrence Seaway, and proceed through the hydrologic network of the Great Lakes to the Ohio coastline of Lake Erie. This value of this measurement is 1,114.10 miles at the Ohio/Pennsylvania border, and 1,304.28 miles at the Ohio/Michigan border. River mile strings of stream locations in the Lake Erie drainage basin always begin with a value between these two numbers.

River mile strings of stream locations in the Ohio River drainage basin always begin with 953.80. This is the distance from the mouth of the Mississippi River to its confluence with the Ohio River. The second value in river mile strings of stream location in the Ohio River drainage basin is a measurement up the Ohio River. The value of this measurement is 490.12 miles at the Ohio/Kentucky border, and 941.40 miles at the Ohio/Pennsylvania border.