Distribution of Mississippi River crest levels per decade, 1900s-2010. NRDC / Data from NOAA

Increased precipitation could increase streamflow by 51 percent, due to more rain falling on near-saturated soils. The risk of floods has already increased over the past few decades along the Mississippi River near St. Louis due to increased flows. Because of its location at the confluence of the Missouri and Mississippi rivers, peak flows at St. Louis tend to be higher than at points north and south of the city. Floods along the Mississippi River tend to last for a month or longer. The number of high Mississippi River crests has generally increased since 1900. For the current decade, which at the writing of this report includes only one year—2010—the number of events approaching flood stage already surpasses the total number of events for each of the first eight complete decades of the 20th century. Indeed, the worst flood to hit the Upper Mississippi River and Missouri River basins occurred in the summer of 1993 after heavy rains fell on already saturated soils. On August 1 of that year, the Mississippi crested at 49.6 feet at St. Louis, just 2 feet short of the St. Louis floodwall. If cresting had been at 52 feet, the St. Louis floodwall would have been overtopped, causing major damage to downtown St. Louis; at 54 feet the Metro East St. Louis and Fish Lake levees would have been overtopped, threatening 71,000 acres of land. As it stood, more than 1,000 levees in the region, most of them agricultural, were topped or breached during the flood. Fortunately, the levees protecting downtown St. Louis and some other metropolitan areas, designed to withstand a 500-year flood, did not fail.

Thirsty for Answers: Preparing for the Water-related Impacts of Climate Change in American Cities