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." With floodwaters expected to crest over the next two days through much of Missouri and Illinois, the corps forecast 18 more levees at high risk of being swamped. In Winfield, Mo., already flooded after a levee was topped, officials pleaded with residents to evacuate. "We are urging, my God, people (in low-lying areas to) get to higher ground," said Cpl. Andy Binder with the Lincoln County Sheriff's Department. As floodwaters moved downriver, President Bush toured parts of Iowa. The floods underscore the need for a national reassessment of the risks that come with development in flood plains across the region, says Charles Dowding, a Northwestern University engineering professor. As wetlands are built up, the river is forced into a narrower channel, "so even if rainfall levels remain constant, the probability of a high flood crest is going to go up." Few of the levees that are being swamped were built to handle a 100-year flood