Source: U.S. Bureau of Transportation Statistics  

Release: Transportation Services Index and Seasonally-Adjusted Transportation Data  

Units:  Millions of Short Tons, Seasonally Adjusted

Frequency:  Monthly

This data is collected by the U.S. Department of Transportation, Bureau of Transportation Statistics (BTS) calculation from U.S. Army Corps of Engineers, Waterborne Commerce Statistics Center, Internal U.S. Waterway Monthly Tonnage Indicators available here.