Humans put considerable effort into controlling river flood water. We build dams to hold the water back, levees to keep it in the river channel, and floodways to divert it from protected areas if the first two reach their limit. The great irony of levees, though, is that the more of them you build, the more dangerous the river becomes. Whatever volume of water comes down the river has to go somewhere. Wall off part of the river’s natural floodplain, and it will have to take up extra space somewhere else.

Climate change looks to be responsible for greater flooding risk in some places, as extreme rainfall becomes more common or weather patterns shift. But greenhouse gas emissions are not the only way we bring more flood damage on ourselves. We also manipulate river systems and construct new developments in risky places.

In late December, a weather system juiced by the El Niño conditions in the Pacific dragged rain across the central US. The Meramec River, which joins the Mississippi on the south side of St Louis, saw about 20 centimeters of rain fall around it over three days. Because a storm a few days earlier had already soaked the ground, most of that rain ran along the surface into the nearest stream. The Meramec River hit record flood stages in the St Louis suburbs, and at least twelve people died in Missouri despite evacuations.

Researchers Robert Criss and Mingming Luo, from Washington University in St Louis, compared the flooding to the previous record—which happens to have come during a similar El Niño-fueled December storm in 1982. If nothing had changed but the volume of water coming down the river, the difference in upstream flow should predict 2015’s higher downstream water levels. But the predicted water levels they calculated were lower than what actually happened in 2015. In other words, it wasn’t just the rain that drove the recent flooding to record heights.

In Eureka, Missouri, the 2015 flood water rose about half a meter higher than could be explained by volume alone. And just downstream in Valley Park, the Meramec River rose a full meter higher. The difference must come down to human changes to the floodplain since 1982, the researchers say. A new section of levee was built near Valley Park in 2005—which was still nearly overtopped by the river. A landfill in the floodplain has increased in size since 1982, eating up space that could have held flood water. New construction on imported dirt also got in the way of floodwaters and added bonus property damage. The water displaced from these areas piled up a little higher everywhere else.

Sprawl has also paved around several small creeks that run through the area and into the Meramec. That turns them into funnels that efficiently catch even more rainwater and quickly add it to the river’s downstream flow. So if you could replay the 1982 storm over the 2015 landscape, the flooding would be worse and result in more damage than it did the first time.

The researchers also looked at the response of the Mississippi River near and downstream from St Louis, where water levels came near or set records. What made this really weird is that it didn’t last long. The Mississippi River, you see, is rather large. Large rivers tend to rise and fall slowly, and high levels usually occur in the spring as snowmelt trickles in from the tributaries. A small stream, on the other hand, can swiftly change character with the arrival of a single rain storm, producing flash floods.

At St Louis, the Mississippi hit the third highest stage on record. The first and second highest occurred around the springs of 1993 and 1973, during periods where the river stayed above flood stage for about 80 to 100 days. From December into January, the Mississippi was above flood stage for just 11 days. It was almost like a flash flood on one of the world’s largest rivers.

The researchers say this is possible because of human manipulation of the river. We work to keep water in the river channel, and so it must go up. And as more rain is captured and sent flying downstream, the Mississippi can sometimes act as wild as a much smaller river.

Separately from any climate trends, some of our actions shift around or magnify flooding risks. Looking forward, the researchers don’t mince words: “Forthcoming calls for more river management, including higher levees and other structures, must be rejected. Additional ‘remediations’ to this overbuilt system will only aggravate flooding in the middle Mississippi Valley[…] New commercial and residential developments in floodplains are foolhardy.” Past behavior predicts we’ll probably do all those things.

Journal of Earth Science, 2016. DOI: 10.1007/s12583-016-0639-y (About DOIs).