Navigable rivers were extracted from the CIA World DataBank II, which is a collection of world map data, consisting of vector descriptions of land outlines, rivers, and political boundaries. It was created by the U.S. government in the 1980s and has a nominal scale of 1:5,000,000.
Travel speeds: Navigable rivers, 20km/hr = 3 min per km.
Source: http://www.evl.uic.edu/pape/data/WDB/
Further information: Gorny, A. J., and R. Carter. 1987. World Data Bank II General User's Guide, Central Intelligence Agency, Washington, DC, USA.
