Congress faces decisions about prioritizing lock construction projects on the inland waterway system ( Figure 1 ). It has authorized 20 such projects for construction, and has authorized preconstruction studies of one other. As both houses debate differing versions of water resources and development bills ( S. 2800 , H.R. 8 ) and the FY2019 Energy and Water Development Appropriations bill ( S. 2975 , H.R. 5895 ), the decision about which of these projects could be undertaken first will likely be among the most controversial issues. In recent years, a single project, the Olmsted Lock and Dam project on the Ohio River, has absorbed much of the available funding. As the time and cost to complete the project have far exceeded initial estimates, other lock construction has been deferred. The Olmsted Lock and Dam is expected to be completed during FY2019, making it likely that funds will be available to begin some, but not all, of the other 21 lock projects a federal advisory board has identified as priorities. Consideration of which projects should move ahead comes at a time when traffic volumes on inland rivers are changing significantly, likely reducing the economic benefits of some potential projects while increasing the benefits of others. The fuel-taxed inland waterway system consists of 11,000 miles of navigation channels with 186 lock sites and 225 lock chambers. River lock sites typically include a dam. This system is designed to allow for barge transportation, which is particularly useful for transporting heavy raw materials (e.g., grain, coal, petroleum, construction aggregates). On the network shown in Figure 1 , barge operators pay a $0.29/gallon federal fuel tax that covers half the cost of new construction and major rehabilitation projects (defined as lock repairs costing more than $20 million) through a special fund in the Treasury, the Inland Waterways Trust Fund (IWTF). The other half is covered by general funds. Over the last five years, the trajectory of cargo volume through the locks under consideration has changed significantly. These recent changes often post-date the initial analyses that were conducted to calculate the expected economic benefits of the individual lock improvement projects. As individual lock construction projects can take many years, the U.S. Army Corps of Engineers (USACE), which oversees the inland waterways and supervises lock construction, requires that the costs and benefits be reevaluated periodically. The Office of Management and Budget (OMB) will not request funding for a project unless the measured economic benefit is at least 2.5 times the expected cost. The 21 proposed projects are clustered in three regions, each facing different economic conditions that are affecting barge traffic in the agricultural heartland, record corn and soybean harvests have reversed the long-term downward trend of cargo volumes on the Upper Mississippi River (seven locks); in the Ohio and Tennessee River Valleys, the closure of coal-fired generating plants has reduced the demand for moving coal by barge to power plants by nearly half (eight locks); along the Texas and Louisiana intracoastal waterway, tank barge traffic is in a state of flux as the petrochemical industry makes longer-term investments related to the Texas shale oil boom (six locks). Additional factors are relevant when considering project priorities. Preliminary work has already been completed on some of the projects (see Table 1 ). For instance, a cofferdam (a temporary concrete wall) has been built around one planned construction site so that the river water can be pumped out and construction of a new lock can take place on dry ground. In some cases this preparatory construction work is already under way or completed, and represents $200 million to $400 million of invested capital. Another consideration is the physical condition of a lock and the likelihood of a major failure that could delay river commerce or even shut it down when a lock must be closed for repair. A related consideration is a lock's economic importance—how much traffic transits the lock and the commodities involved. Additional capacity is a motivation underlying many lock projects. Congress authorizes new navigation construction projects, modifies existing projects, and sometimes deauthorizes projects in periodic water resources bills. It also sometimes uses a Water Resources Development Act (WRDA) as a means to require reviews or broader changes in Army Corps policies. After a lock project is authorized, construction can begin only when funds have been appropriated. Party rules in both houses that prohibit earmarks restrict legislative proposals that would direct a specific amount to a specific project. A standard size barge is 195 feet in length and 35 feet in width, and can hold up to 1,570 tons of cargo. On the busier, main stems of the locked segments of rivers, 15 barges are tied together in a single tow, usually three barges wide and five barges in length. The barges are pushed by a towboat (also known as a pushboat) that is typically between 117 feet and 200 feet in length. Thus, a 15-barge tow is about 105 feet wide and can be just under 1,200 feet in length. The largest locks on the system, which are 1,200 feet by 110 feet, can accommodate these tows. However, many of the locks are half this length (600 feet), requiring a 15-barge tow be broken up into two sections to pass through a lock. One lockage can take 45 minutes to an hour as the barges enter the lock chamber, the chamber is filled or emptied, and the barges exit. Thus, a 15-barge tow attempting to pass through a 600-foot lock will require two lockages taking about two hours. After it pushes both segments of the tow through the lock, the towboat will reassemble them into a single tow. On less busy tributaries, some locks are 360 feet by 56 feet, large enough for only one barge at a time to lock through. However, on these river segments, tows may consist of only four or six barges. The width and curvature of rivers also dictates allowable tow sizes. The 15-barge tows are most often carrying grain or coal. Tows carrying other dry goods often consist of a smaller number of barges and may contain a mix of commodities. Tows of tank barges carrying liquid cargo (petroleum, chemicals) typically consist of one to three barges. "Jumbo" barges whose dimensions are 290 feet x 50 feet can also be found on the system. The Mississippi River between St. Louis and New Orleans has no locks, and 30 to 50 barges are often tied together in tows. One of these tows carries about as much cargo as an oceangoing ship. Tows travel about 3 to 5 mph. On multiday voyages, barges operate day and night with two crews rotating six-hour shifts. A voyage between Pittsburgh and St. Louis will take about 18 days, a voyage between Peoria, IL, and New Orleans about 16 days. Both drought and floods may limit the size of tows or restrict transport to daylight hours only, causing lengthy delays. Some northern portions of the system are either closed entirely in winter or are open but susceptible to delays due to ice, especially around locks. Winter closure allows the Corps to perform major lock repairs without disturbing traffic, although not under ideal weather conditions. The nature of river transport generally caters to shippers of low-value commodities that ship in large lot sizes and can afford to stockpile their product in anticipation of unfavorable river conditions. The main competition for barges is railroads, which cost more but generally provide faster and more reliable transport. For petroleum products, pipelines are the predominant choice on specific routes, but are costly to alter if origin or destination changes. Most barge shippers are committed to river transport, especially insofar as they have located their facilities along a river for this purpose and cannot readily switch to rail or pipeline transport when there are problems with river transport. The Army Corps of Engineers maintains a depth of at least 9 feet throughout the inland waterway system. This depth is accomplished by dams that create pools (lakes) and that moderate river currents and water supply, as well as by dredging. Deepening is not an element of most of the lock improvement projects. Various "training structures" also funnel the current and help prevent shoaling in the middle of a river. The Corps operates locks 24 hours a day, seven days a week at locations with at least 1,000 commercial lockages per year (an average of about three per day). Below this level, locks may be closed at night. Reliability has been a persistent issue, with both scheduled and unscheduled repair work requiring barge operators to work around lock closures or utilize a typically smaller auxiliary lock. While age is frequently cited as justification for constructing a new lock, many locks have undergone major rehabilitation since their initial construction. The rehabilitation work is not always reflected in USACE reports. Barge operators and agricultural shippers in the upper Midwest have long sought to increase the size of a series of locks above St. Louis (Locks 20, 21, 22, 24, and 25) on the Mississippi River, as well as the last two locks on the Illinois Waterway (Peoria and LaGrange). This project is now referred to as the Navigation and Ecosystem Sustainability Program (NESP; see Figure 1 ). Corn and soybeans, and to a lesser extent, wheat, are the dominant commodities in terms of tonnage shipped through these locks (see Figure 2 and Figure 3 ). Once on the river, these products are typically bound for the New Orleans region to be loaded on ships for export. Fertilizer, moving upbound, is also a significant commodity being shipped through these locks. All seven of these locks are currently 600 feet in length and 110 feet in width, which means that the 15-barge tows typically found on the river moving grain for export must be split into two sections to pass through.