The United States' inland waterways system — more than 19,000 kilometers (12,000 miles) of navigable routes maintained by the U.S. Army Corps of Engineers overlaid with expansive farmlands — has contributed greatly to the country's success . Ongoing use of the waterway system requires the maintenance of infrastructure to meet usage demand, including dredging of ports and rivers, and the operation and maintenance of dams, levees and locks. U.S. Inland and Intercoastal Waterways The Mississippi and Ohio rivers and the Illinois waterways, the busiest avenues for commercial traffic on inland waterways, all have expansive lock systems. The locks make navigating a river easier, sequestering vessels before raising or lowering the water level in a chamber in order to compensate for changes in the river's level. Most of these locks were constructed in the early 20th century, with an expected lifetime of 50 years.