The inland waterways are a critical component of the nation's freight transportation system-moving over $230 billion of coal and other bulk commodities in 2014. Vessel operators that transport these products via barge pay a $0.29 per-gallon fuel excise tax, which provides partial funding for new construction and major rehabilitation of navigation infrastructure, such as locks and dams, on the waterways. From 2005 through 2014, the fuel tax generated about $83 million per year. Any non-compliance, such as not filing or under-reporting the tax, would reduce available funding. The Water Resources Reform and Development Act of 2014 included a provision for GAO to evaluate the efficiency of collecting the fuel tax and assess alternative collection methods. This report describes: (1) the results of IRS's efforts to ensure compliance from 2005-2014; (2) challenges IRS and inland waterways operators report in determining taxes owed; and (3) alternative methods of collecting the fuel tax under existing law, and their advantages and disadvantages. GAO analyzed IRS audit data from 2005- 2014, the most recent available data; reviewed relevant tax policy papers and reports; and interviewed Corps, IRS, and Treasury officials, as well as a non-generalizable sample of 10 vessel operators to obtain perspectives on fuel tax compliance.