The historical record includes marked shifts in the appreciation of rivers, numerous conflicts in use demand, and an intensification of use that rapidly accelerated during the 20th century. External freight trade became concentrated in estuarine ports rather than in inland ports when oceangoing vessels increased in size. Even the port of London, though constrained by high capital investment, has displaced itself toward its estuary. The Amazon remains naturally navigable by ocean ships for 3,700 kilometres (2,300 miles), the Yangtze for 1,000 kilometres, and the partly artificial St. Lawrence Seaway for 2,100 kilometres. Internal freight traffic on the Rhine system and ...(100 of 32811 words)