Background
Climate alarmists claim that both floods and droughts will become more frequent and/or severe in response to global warming.  Hence, it is instructive to compare the properties of these episodic phenomena over the past century or so with their characteristics over the preceding centuries, as climate alarmists also claim that the warming of the 20th century was unprecedented within the context of the past two millennia.

Providing special urgency for the task is the fact that broad areas of the US Northern Great Plains experienced notable lake highstands during the 1990s.  One of the most affected areas was the Waubay Lake complex in eastern South Dakota, which rose by 5.7 meters and more than doubled in area from 1993 to 1999, severely flooding roads, farms and towns and prompting the Federal Emergency Management Agency to declare the region a disaster area on 1 June 1998.

What was done
To determine the historical context of the rising lake levels, the authors developed a 1000-year hydroclimate reconstruction from local bur oak (Quercus macrocarpa) tree-ring records and lake sediment cores, analyses of which involved the shell geochemistry of the ostracode Candona rawsoni.

What was learned
In the words of the researchers, "prior to AD 1800, both lake highstands and droughts tended towards greater persistence than during the past two centuries," such that "neither generally low lake levels occurring since European settlement (but before the recent flooding) nor the post-1930s pattern of steadily increasing water availability and favorableness for tree growth are typical of the long-term record."

What it means
In this particular part of the world, longer-lasting floods and droughts of equal or greater magnitude than those of modern times occurred repeatedly prior to 1800.  Hence, one must search elsewhere for evidence that would support climate-alarmist claims of more frequent and/or severe floods and droughts occurring in response to global warming, for there's none to be found here.