Free S57 US Inland ENCs
 

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US Inland Electronic Navigation Charts -
The US inland navigation system consists of 8,200 miles of rivers
maintained by the Corps of Engineers in 22 states, and includes 276
lock chambers with a total lift of 6,100 feet. Following
recommendations by the National Transportation Safety Board, the
National Academy of Science and the American Waterways Operators,
Congress directed the Corps of Engineers to develop and publish
electronic charts for the inland waterways.
http://www.tec.army.mil/echarts/inlandnav/main.html

Gerald