Why the Low Water Levels on the Mississippi River

There has been much concern with the low river stages on the Mississippi River between St. Louis and Cairo, IL, and what that means for barge traffic, for example here and here.
How bad is it? The first graph shows the river stage at Chester, IL, from January 2011 to present. The river was actually above flood stage for much of May-July 2011. It dropped from almost 40 feet during that time to 5 feet in November 2011. Then it fluctuated between 5 and 25 feet, until this summer when it dropped below 5 feet and into negative numbers in late September. The latest forecast from the NWS has the river stage reaching -2 feet by December 10 (second figure). By the way, you can get negative river gauge heights because the reference point on the gauge (zero) may not always be the bottom of the river.
The reason for the low river stages in the stretch between St. Louis and Cairo is that much of the Upper Mississippi River basin and the entire Missouri River basin are in some stage of drought, according to the U.S. Drought Monitor (last figure). For the Missouri River basin, 90 percent of the basin is either abnormally dry or in some stage of drought. In fact, 41 percent of the basin is in D3 or D4, the two worst categories of drought.
Conditions are just as bad on the Upper Mississippi with 96 percent of the basin either abnormally dry or in some stage of drought. However, only 17 percent of the basin is in D3 or D4 drought. So drought is a little more widespread but not as severe in the Upper Mississippi River basin as it is in the Missouri River basin. Unfortunately, the latest NWS drought outlook indicates that the drought is expected to continue across much of these two basins through this winter.

Table 1. Percent coverage by USDM drought stage for each basin as of November 20, 2012. Data courtesy of the National Drought Mitigation Center. 
None D0-D4 D1-D4 D2-D4 D3-D4 D4
Missouri River 10.3 89.7 82.8 67.1 41.2 21.9
Upper Mississippi River 4.4 95.6 76.9 34.9 16.5 0

Mississippi River levels at Chester, IL. Source: USGS. Click to enlarge.

NWS forecast of the Mississippi River stage at Chester, IL, through December 10.

U.S. Drought Monitor map for November 20, 2012. Click to enlarge.

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