At least 18 more levees on the already flooded Mississippi River are at high risk of being overwhelmed this weekend, endangering small communities and farmland where decades-old flood protections are far below modern-day guidelines.

At least half of the 31 levees already breached or topped between southern Iowa and St. Louis were not built to handle a flood of such historic proportions, according to a USA TODAY review of data from the Army Corps of Engineers. Many of those were built at least 30 years ago, and some date to the 1940s.

As riverside development has boomed, "levees built 50 years ago for agricultural purposes are often now asked to do the work of a residential or urban levee," says Eric Halpin, a corps special assistant for dam and levee safety. The floods "show a need for more robust and resilient levee systems."