Dam safety incidents can occur at any point during a dam’s lifetime, but the most common period of dam failure is the first 5 years of operation. The United States Society of Dams conducted a study in 2009 of 1,158 national and international dam failures and safety incidents and found that 31 percent of safety incidents occur during construction or within the first 5 years of operation. The most common causes of failure are overtopping, piping, and foundation defects. Overtopping caused by flooding and high-water events accounts for 34 percent of dam failures in the United States. Erosion caused by overtopping can compromise embankments and lead to failure. The risk of overtopping increases if the spillway design is inadequate, debris causes spillway blockage, or the dam crest settles.

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Most locks are designed to last for 50 years, but 54 percent of IMTS locks are more than 50 years old, and 36 percent are more than 70 years old. Many of these locks are in need of repair and replacement, and some lack basic maintenance. Concrete is crumbling at some locks, and some have not been painted in 25–30 years, increasing the risk of corrosion. Locks lacking maintenance or in need of repair and replacement are more likely to have unscheduled closures. Unscheduled closures are more costly than scheduled closures, because vessel operators and companies are unable to plan to offset the delays from these incidents. The annual number of unscheduled lock closures has steadily increased since 1992. Fewer than 7,000 unscheduled closures occurred every year before 2000, and more than 7,000 occurred every year after 2000, peaking in 2008 with 13,250.

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