More U.S. merchandise trade (measured in tons) is carried by oceangoing vessel than by airplanes, trucks, freight trains, and pipelines combined. About half of U.S. merchandise trade by value and nearly 80% by volume enters or leaves the United States through a seaport. Harbor maintenance thus has a critical role in facilitating U.S. foreign trade. It is also important in minimizing the cost of trade between the continental United States, Alaska and Hawaii, and U.S. island territories. A limited amount of coastal shipping (e.g., Houston to Tampa) also relies on waterways and harbor channels maintained by the U.S. government. Congress has chosen to make the U.S. Army Corps of Engineers (the Corps or USACE) responsible for maintaining federal navigation channels. The Water Resources Development Act (WRDA) is the principal legislative vehicle for altering the Corps' Civil Works Program; its consideration offers Congress an opportunity to revisit harbor maintenance policy. In 1986, when Congress established current harbor maintenance policy, merchandise trade accounted for 14% of GDP. It now accounts for 25%, so decisions on harbor maintenance may have greater impact on the U.S. economy than was the case in 1986. Under existing law, the cost of harbor maintenance is supported by a harbor maintenance tax (HMT) on imported and domestic waterborne cargo and cruise passengers. The tax rate is 0.125% of the value of cargo shipped or cruise tickets sold, representing $1.25 per $1,000 of cargo or cruise ticket value. The tax is generally assessed at coastal and Great Lakes ports. Almost all of the collected revenue is generated by imported waterborne cargo. Revenue from the tax flows into the harbor maintenance trust fund (HMTF), which is used to cover the Corps' cost of dredging channels, maintaining jetties and breakwaters, and operating locks along the coasts and in the Great Lakes. Unlike some other federal trust funds, such as the highway trust fund, the HMTF may be drawn on only with an appropriation by Congress. Under congressional budget rules, HMT revenue is scored as federal receipts, and expenditures from the HMTF are scored as current-year expenditures; thus, a surplus of HMT receipts over expenditures reduces the federal budget deficit. In recent years, a little more than half of the HMT collected annually has been spent on harbor maintenance. Unexpended HMT collections remain in the trust fund, and the fund balance is credited with interest from the Treasury. The balance in the trust fund was $8.5 billion as of July 2013. In FY2014, the Corps expects the HMTF to receive $1.8 billion from the harbor maintenance tax and $247 million of interest, for total income of $2.05 billion. The Administration's budget requests $923 million for harbor maintenance. Bills in the 113 th Congress, S. 601 (approved by the Senate in May 2013), S. 218 , and H.R. 335 , if enacted, would nearly double annual spending on harbor maintenance and related activities, requiring that HMT revenues be spent for harbor maintenance rather than accumulating in the trust fund account. H.R. 3080 , introduced by the leadership of the House Transportation and Infrastructure Committee and Subcommittee on Water Resources and Environment, would increase HMTF expenditures every year, so that by FY2020 and thereafter, 80% of the HMT collected the prior fiscal year would be spent on harbor maintenance. Increasing spending from the HMTF has the potential to reduce funding for other Corps activities, such as harbor construction or flood control, or other activities funded under the Energy and Water Development Appropriations acts. Congress could compensate for increased funding from the HMTF by allocating more funds to the energy and water development appropriation, but this would require less funding for activities outside this appropriation to stay within total federal budget caps. When Congress established the HMTF in the Water Resources Development Act of 1986 (WRDA 1986; P.L. 99-662 ), it specified that the funds were to be used to pay for "the eligible operations and maintenance costs assigned to commercial navigation of all harbors and inland harbors within the United States." Congress defined "harbors," in part, as "capable of being utilized in the transportation of commercial cargo in domestic and foreign waterborne commerce by commercial vessels," and defined "inland harbors," in part, as "used principally for the accommodation of commercial vessels and the receipt and shipment of waterborne cargoes on inland waters." A key distinction in these definitions is that "inland harbors" must be currently handling cargo, while coastal or Great Lakes harbors need only be "capable" of handling cargo. In both definitions, "commercial cargo" includes passengers transported for compensation but excludes fresh-caught fish. Ferries are excluded from the definition of "commercial vessel." These definitions have not been substantively changed since 1986. The Corps did not conduct a rulemaking to implement the harbor maintenance provisions in WRDA 1986, so there is no Federal Register record of its interpretation of the act. According to its annual reports to Congress on the status of the HMTF (the most recent of which covers FY2006), the Corps interprets the term "commercial navigation" to mean any project authorized by Congress with commercial navigation as an authorized purpose. According to the Corps, most harbor projects are authorized as "single-purpose" commercial navigation projects, and therefore all operations and maintenance (O&M) costs at those harbors are payable from the HMTF. If a project is authorized as a "multi-purpose" project, aiding commercial navigation along with purposes such as recreation or flood control, only expenditures on behalf of commercial navigation and joint-use costs allocated to commercial navigation are eligible for recovery from the HMTF. Based on information Corps personnel recently presented to the ports industry, the 59 ports the Corps defines as "high use"—those handling more than 10 million tons of freight per year—accounted for 90% of maritime commerce (by tonnage). These ports received approximately two-thirds of HMTF spending in FY2013. Almost all the high-use ports had operations and maintenance (O&M) activities scheduled (see Table 1 ). "Moderate use" ports, those handling between 1 million and 10 million tons of cargo, accounted for 9% of maritime commerce but received 19% of HMTF spending in FY2013, and about half had O&M projects planned. Some 7% of "low use" ports had federally funded O&M projects planned; the low-use ports collectively handled 1% of all maritime commerce, but received 5% of HMTF spending. A roughly similar pattern prevailed in FY2012, according to the Corps. This table encapsulates an important part of the HMTF debate, also reflected in differences between S. 601 and H.R. 3080 , over how HMTF funds should be allocated. Seen from one point of view, the vast majority of harbors—those classified as "low use" or "moderate use"—receive little or no federal funding, perhaps limiting their ability to receive cargo, fishing, and recreational vessels, and thereby damaging local economies. Seen from another point of view, harbors used by big commercial vessels, which generate almost all HMT revenue, receive a smaller share of HMTF spending than their economic importance justifies, while a disproportionate share of funding goes to harbors that receive little or no commercial traffic. H.R. 3080 specifically directs the Corps not to allocate harbor maintenance funds solely according to tonnage, while S. 601 appears to favor funding of large, commercial cargo ports. Corps data provided to CRS identify a total of 1,228 waterways or segments of waterways on which operations and maintenance are eligible to be reimbursed by HMTF funds. Of these, 752 waterways or segments (61% of the total) carried no cargo from FY2005 to FY2009, and 100 handled less than one truckload of cargo per day (8,030 tons per year). An additional 50 waterways eligible for HMTF spending handled less than the equivalent of one Panamax shipload per year (52,000 tons or about 2,400 truckloads). Thus, almost 75% of the eligible waterways handled little or no cargo. While the Administration's budget requests for the Corps in recent years have directed the majority of HMTF spending to high-use harbors, "high use" is defined by the volume of cargo, as measured in tons, rather than the value of cargo that is the basis of the tax. The volume-based metric tends to favor ports that handle tankers carrying oil and chemical products and bulk ships that carry grains, ores, and coal. Alternatively, if a value-based metric were to be used, budgeting might direct a greater share of HMTF funds to ports that handle container ships, whose cargo is of much greater value on a per-ton basis. This would tend to concentrate harbor maintenance funding, as the five largest container ports—Los Angeles, Long Beach, New York/New Jersey, Savannah, and Oakland—collectively handled about 56% of U.S. maritime container shipments in 2012. Ports defined by the Corps as "high use" received only about half of the O&M funds requested by their Corps district engineers in FY2011. (It is not known to what extent the district engineers' requests represent a genuine "needs list" versus a "wish list.") Moderate-use ports received about 30% of the funds their district engineers requested, and low-use harbors received 10%. The total amount requested for low-use harbors, $766 million, nearly approached the amount requested for the busiest harbors, $922 million.