About 26,000 U.S. dams pose a high or significant hazard to life and property if a failure occurs

NBF - more than 80,000 dams in the U.S., about a third pose a "high" or "significant" hazard to life and property if a failure occurs, according to the Federal Emergency Management Agency (FEMA).

What are the chief causes of dam failures in North America?
Overtopping is the highest and seepage and piping is probably second

Overtopping is something that's avoidable. You can perform the necessary calculations and develop a "fix" to prevent it. Overtopping might occur because the spillway was inoperable. That's the type of thing that causes dams to overtop in general, other than extreme floods.