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The Flood of 2008–December

It happened again on Saturday: Warm moist air hit a cold front and dumped inches and inches of rain, two inches in two hours in west-central Illinois, melting ice and snow and bringing flooding once again to the  Midwest. The Mississippi rose 2.83 feet at St. Louis on Saturday, 9.68 inches on Sunday, and 4.o5 inches on Monday. Authorities handed out sand bags along streams in the region around Chicago.

In November I attended a conference at St. Louis University that addressed Midwest flooding and global climate change. 

Zaitao Pan, from the Department of Earth and Atmospheric Sciences at the university, detailed that the temperature of the surface air over the globe rose 1º F during the 20th century, the fastest rate of warming in the last 2,000 years.  The central U.S. has experience less warming than other parts of the world, particularly in the summer when the mean temperature has decreased in some areas. 

But, this has led to more rainfall over the Midwest when precipitation has increased about 6% in the 20th century, and 20% since 1960, compared to 2% elsewhere.  And, it  has increased the intensity and frequency of heavy rains, like that of Saturday. The number of days with rainfall of over 4 inches has increased 50% since the beginning of the 21st century.

If the climate continues to warm, the atmosphere will hold more moisture, leading to more big storms and more flooding in the Midwest with a 21% increase precipitation by mid-century, leading to a 51% increase in runoff and a 43% increase in groundwater recharge, resulting in a 50% increase in total water yield in the Upper Mississippi River Basin, that is more flooding.

 

Kiser Creek

Sny Island: Kiser Creek, 1995

Alterations to the landscape speed flooding to the Mississippi. Take Sny Island in Central Illinois south of Quincy. We call it Sny Island because the Sny River runs yazoo-style between the bluff and the Mississippi and delivers runoff from the floodplain to the big river. At one time the Sny broke off from the Mississippi at its head and returned to the Mississippi at its mouth.

In the wake of  a heavy rain, Kiser Creek spills out of the uplands at New Canton, Illinois, crosses over the Sny River on an aqueduct, and empties into the Kiser Creek Diverson Ditch. Corseted between levees, it runs straight as a string across Sny Island  and delivers its water and sediment into the southern end of Murphy Bay, a side channel of the Upper Mississippi. The Kiser Creek Ditch is one of three built on Sny Island in the 1960s.

The completion of Locks and Dams 22 and 24 on the nine-foot navigation channel on the Upper Mississippi  raised the water level at the mouth of the Sny River, which runs the length of the floodplain. Water backed up into the Sny and into the diverted creeks that emptied into the Sny. Silt clogged the channels. Heavy rains that took four days to drain before the dams took four weeks to drain after. Under the Emergency Flood Control Act of 1950, the Corps of Engineers reconfigured the drainage plan for all of Sny Island at a cost of twenty million dollars.

The Corps channeled five creeks that spill onto Sny Island through three diversion channels than ran from the hills to the Mississippi, crossing over the Sny on aquaducts. The engineers built thirteen reservoirs, catch basins for silt streaming out of the uplands, raised and strengthened twenty miles of Mississippi River levee that protects the floodplain from flooding, dredged excess silt from the Sny, and built three pumping stations along its seventy-three mile course.

After the Corps finished the drainage system in 1967, farmers on Sny Island complained of seepage in their fields, which they traced to the navigation channel. In 1980 the Corps granted a lump sum payment to the drainage districts which allowed farmers to tile-drain their fields and build private ditches that empty into the Sny via districtdrainage ditches.

All of these actions speed the drainage of water, streaming off the uplands and falling on the floodplain, to the Mississippi.
References for this post:
Pan, Zaitao, “Climate Change, Precipitation, Soil Moisture, and Streamflow in the Central United States, Abstract of talk delivered at Finding the Balance between Floods, Flood Protection, and River Navigation, St. Louis University, November 11, 2008.
Gard, William T., The Sny Story, North Richland Hills, Texas: Smithfield Press, 2002. 


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