The Natchez Bluff Is Fragile
“The deterioration of these bluffs has created a profound danger to both life and property. These bluffs overlook the Mississippi River and are formed by loess soil, a very fine powdery substance that practically liquefies when it gets wet.”
Senator Trent Lott to Congress, January 20, 1995
The historic 19th century residential and commercial center of Natchez is built on a vertical, 200-foot bluff overhanging the Mississippi River. The bluff and the earth beneath the town itself are largely composed of a soil type known as “loess,” which is subject to periodic slumps or collapse, landslides, sinkholes and mudflows, especially following rainfall.
Loess is a light, wind-blown glacial deposit that is extremely porous and water-soluble. Remarkably, loess has the unique ability to stack up vertically.
Loess develops vertical cracks, which allow water to penetrate the subsurface. Water percolates through the loess and underlying layers of sand, silt, and gravel.
Past geological studies that have considered the problem of loess soil in the U.S. have concluded that certain areas in Adams County are danger zones; the most prominent such area is the line of high bluffs bordering the Mississippi River.
In the past seven decades, there have been 10 major collapses of the Natchez Bluff, resulting in the loss of 6 lives and 22 acres of property on the bluff’s edge. On the average, Natchez has experienced one major collapse every 7.4 years.
Following a fatal collapse of the downtown Natchez Bluff at Silver Street in 1980, Congress appropriated public monies to fund an extensive investigation by the U.S. Army Corps of Engineers “to identify the causes of bluff instability at Natchez, Mississippi, and provide recommended remedial measures which may be taken.”
- A significant conclusion of the study was that water is a factor in most bluff failures: “Results of the investigation indicate that infiltration of water into the bluff from rainfall and leaky utilities is largely responsible for bluff instability.”
- The Corps found that during the 121-year study period, bluff losses approaching 100 ft had occurred in some areas and that losses on the order of 50 to 60 feet were not uncommon.
- Of great significance: “From the analyses, it was found that the most likely failure surfaces would extend from a point 20-30 feet back from the top of the bluff down through the loess and the Natchez Formation to the toe of the slope.”
Source: U.S. Army Corps of Engineers, Natchez Bluff Study, 1985