Is there an easy method for determining river mile?

I would like to determine the river mile of 25 sites along the Ohio River (USA). I understand that there is an ArcGIS tool for measuring the distance between two points, but I am not sure how accurate this method would be for a river that stretches nearly 1000 miles. Is there an interactive map or GIS data layer that will show river mile by clicking on any point of the river?

ArcGIS

Rivers

Maps

Hydrology

Cartography

2nd Oct, 2015

Baptiste Hautdidier

French National Institute for Agriculture, Food, and Environment (INRAE)

Short comment on the GIS way, in addition to Daniel, Timothy and Mounir : the feature is called linear referencing in ArcGIS. See here for an explanation and a tutorial: http://resources.arcgis.com/EN/HELP/MAIN/10.2/index.html#//003900000001000000

You should be able to convert your hydrological dataset (here restricted to the Ohio river + its tributaries if you wish) as a route feature class. If I remember well, suffice then to define the point layer with your sites as events, by locating features along routes: http://resources.arcgis.com/EN/HELP/MAIN/10.2/index.html#/Locating_features_along_routes/003900000037000000/

The computation will still depend on the coarseness of the dataset, which is one reason why the precise length of a large river remains often a disputed (and nearly elusive) question... but it's another story!