The Midwest flooding is being called a 500 year event, but some hydrologist believe that the effects of heavy spring rains were enhanced by the drastic changes in the landscape wrought by 150 years of intensive agriculture. In the past the tall grass prairie and an interconnected web of wetlands, streams and rivers drained east central Iowa into a Mississippi River unconstrained by levees. The great river routinely exceed its banks, cut new channels, and created lakes along its flow. The natural geography acted as a sponge, soaking up heavy rains, collecting, storing and directing the water south to the Gulf. The Des Moines lobe, a region of fertile wet prairie in north central Iowa was described by Rufus Blanchard in 1868: "the country consists of broad tables with but slight depressions, drained by frequent undulating sloughs. These are found in parts most distant from large rivers, where the admirable system of drainage which nature has provided for this State has not yet had time to be developed."[1] But that landscape is gone forever because the natural attributes of the land were not adequate for the needs of mechanized farming however. A mono-crop agricultural landscape extends as far as the eye can see. For nearly a century and a half, Midwest farmers drained, dredged, and tiled the wetlands and marshes on the Des Moines Lobe and across the greater Prairie Pothole Region. Estimates generally agree that approximately 99 percent of the original wetlands, marshes, and small streams of north-central Iowa were drained and plowed.[2] The effect of all this human hydrology has been to remake Iowa into a vast tilted plain much more permeable to rain water, allowing greater volumes of water to enter streams and rivers than historically possible. The Cedar River broke it's previous high water mark by eleven feet after 15 inches of rain was dumped on the central and eastern parts of the state. The ground was already saturated from previous precipitation. So most of that water entered the man made drainage courses causing area rivers to reach record stages. The U.S. Geological Survey has preliminary data showing 500-year floods on the Cedar, the Shell Rock, the Upper Iowa and the Nodaway rivers. The 1993 flood was also considered a 500 year event. 
The anomaly of two statistically unlikely events with 15 years started meteorologists thinking. Some attribute the flooding to cyclical climate change. The Midwest has been in a wet cycle for the past 30 years. Others are willing to attribute the severe weather to global warming without direct evidence. Computer climate models do predict more severe weather as a result of global warming. A recently released government survey of academic literature on the subject predicted a 90% chance that the frequency and intensity of heavy downpours will rise. In official understatement the report opined: “…on balance, because systems have adapted to their historical range of extremes, the majority of the impacts of events outside this range are expected to be negative.” Pass me a sandbag, dude.[1]http://www.igsb.uiowa.edu/Browse/histalt/HISTALT.HTM
[2] id. Image of a mole plow used to drain land.