The Mississippi River has had more extreme floods during the past 100 years as a result of engineering projects, according to a new study published in the journal Nature that looked at the river’s flood history over the past 500 years.

The research, led by scientists at Woods Hole Oceanographic Institution, found that during the past 150 years, the number of 100-year floods have increased by 20 percent. A 100-year flood is one that has a 1 percent chance of occurring in any given year.

“The floods that we’ve had over the last century are bigger than anything we’ve seen in the last 500 years,” Sam Munoz, lead author of the study, stated.

The main causes for the increase was found to be “federally funded river engineering projects that began after 1928 to facilitate commercial navigation on the river and to protect communities and cropland from floods,” the institute stated on its website. These include projects to straighten the river, create channels, and the construction of artificial levees.

The researchers performed the study by extracting 30-foot-deep sections of earth underneath oxbow lakes—former sections of the river that had been disconnected and became sitting bodies of water.

On its website, the institution explained why these portions are indicators of the river’s flooding history: “During large floods, faster moving water from the river channel stirs up larger-grained sediments and flows into the usually disconnected lakes, carrying sediments and debris along with it. The material from the river is caught in the lakes and eventually sinks. It forms a layer on the bottom that is subsequently buried over time. The layers are telltale clues of past floods. The deeper the cores, the further back in time the scientists can reach.

“The grain size in the layers provides clues to the size of the floods. The larger the flood, the more energy is generated by the river water, and the larger the grains that get deposited in the lakes, said WHOI geoscientist Liviu Giosan, another lead member of the research team. By analyzing grain size and flood size for known flood events—for example, the Great Mississippi River flood of 1927—Munoz could estimate the sizes of previously unknown floods represented in the sediment cores.”

It was noted that 25 percent of the major floods were caused by natural periods of heavy rain. As for the other three-quarters: “The obvious culprit is that we have really modified the river itself,” Dr. Munoz stated according to Science News.