News•December 18, 2012

A Year After Flooding, Commerce on Mississippi Imperiled

The drought that continues to affect a majority of the lower 48 states is jeopardizing the flow of commerce along parts of the Mississippi River, a vital waterway for transporting $7 billion worth of commodities such as coal, grain, cement, chemicals and other materials.

Hydrograph from the gauge at the Mississippi River at St. Louis, MO, showing the predicted fall to near the record low of -6.2 feet.
Click to enlarge the image. Credit: NOAA.

It was only a year ago that there was record flooding along the Mississippi River and many of its tributaries. Heavy rainfall combined with spring snowmelt to cause the mighty Mississippi to overflow its banks, damaging towns and farmland that line the waterway.

But this year the drought that has engulfed the U.S. has taken its toll. As of December 11, 61.87 percent of the land area in the continental U.S. was still under some degree of drought, according to the U.S. Drought Monitor, with the most severe impacts concentrated in the High Plains, Texas, and parts of the Southeast.

The Mississippi River has not been immune to that. The river at St. Louis is close to setting an all-time record low, although storm systems moving through the Plains and Midwest — including a major snowstorm expected Wednesday into Thursday — are likely to drop enough rain and snow to prevent the record from occurring during the next week.

The seesaw from too much to too little water may be a sign of what’s to come as manmade global warming intensifies the water cycle, leading to more precipitation extremes, both heavy rains and drought events.

Historically, the winter lows for streamflow at St. Louis occur in late December through January, according to Steve Buan, a hydrologist with the National Weather Service in Chanhassen, Minn. According to NOAA data, January-November 2012 ranked as the third-driest such period on record for the Mississippi River basin north of Memphis, behind the Dust Bowl years of 1936 and 1934.

At the New Madrid gauge in New Madrid, Mo., the Mississippi reached a record high of 48.35 feet on May 6, 2011. Just 15 months later, on Aug. 30, 2012, the gauge reading dropped to a record low of minus 5.32 feet. (River gauges are calibrated to a particular elevation, known as a “zero datum,” which means that they don’t always equal the depth of water in the channel. So in this case, the record low was 5.32 feet below the zero-datum elevation at New Madrid.)

July to October precipitation departures from average across the Mississippi River Basin.
Credit: NOAA.

The New Madrid gauge is downstream from where the Ohio River empties into the Mississippi, and it is currently running close to 14 feet, which is not problematic for shipping.

In other parts of the river, though, water levels are already low enough that shippers have had to cut back on the number of barges that they are running, as well as the amount of goods on each barge. The low flows north of the intersection of the Ohio River and the Mississippi also have the potential to stymie river borne commerce altogether.

In particular, the approximately 180-mile stretch of the Mississippi River between Cairo, Ill., and St. Louis is of the most concern for low-water levels, according to Victor Murphy, climate services program manager for the National Weather Service’s Southern Region in Dallas. The low water in the area is in stark contrast to 2011, when the Army Corps of Engineers was forced to blow up a levee near Cairo, flooding farmland, in order to save the town from devastating flooding.

One especially treacherous low-water section of the Mississippi is currently located near the town of Thebes, Illinois, where submerged rocks, known as “pinnacles,” jab toward the surface of the river, threatening to ground passing vessels.

January through November precipitation across the northern Mississippi River Basin.
Click to enlarge the image. Credit: NOAA.

On Monday, the Army Corps announced plans to release more water from Carlyle Lake in Illinois to aid in safe navigation along the Mississippi near Thebes, and the agency is also planning to blast submerged rocks and conduct dredging operations to keep barges and ships from running aground.

“Water from the lake will help provide the depth necessary for river commerce to pass Thebes, Illinois, where rock formations pose a risk to navigation at minus 5 feet and below on the St. Louis gage,” the Corps said in a press release. The Corps said the water releases will provide an additional six inches of depth in this critical stretch of the river.

Even with such assistance, many see at least a temporary closure of this section of the river as all but inevitable. The latest two-week outlook for the Mississippi River at St. Louis shows that the river is likely to drop to levels last seen in 1989, and may come close to setting an all-time record low. The current record low river level stands at minus 6.2 feet.

The Upper Mississippi River watershed, which is comprised mainly of the states of Illinois, Iowa, Minnesota, Missouri, and Wisconsin, have seen below-average precipitation for many months. That was especially the case during the July-through-October period, when precipitation averaged 4-to-8 inches below average. The same period in 2011 was also extremely dry. The four-month period is a key time when there is increased water demand for crops and a high rate of evapotranspiration from plants and soils. Below-average rainfall during this critical period has created an enduring soil moisture deficit, which has not been overcome.

“For two successive growing seasons, the crops have had to draw on the soil moisture bank account to make up for the lack of precipitation to produce the grains. That soil moisture is what would normally be feeding the ‘base’ river flow at this time of year,” Buan said in an email conversation.

At this point, rain and snowmelt can add to river flow during the short-term, but with colder weather forecast for late December into early January, the chances for a significant recovery to more typical river levels are considered remote.

“If one of these storms is followed by a protracted and severe cold temperature outbreak, that will further reduce streamflow as water that is already in the river freezes and is no longer available to move downstream. We call this reduction in streamflow ‘ice bite,’ ” Buan said.

Long-range weather outlooks show that after a mild start to the winter, Arctic air masses may move into the U.S. starting in late December, leading to below-average air temperatures. That would also tend to favor even lower river levels.