Dredging of the Fractured Bedrock-lined Mississippi River Channel at Thebes, Illinois
A recently-published article by Kenneth R. Olson and Lois Wright Morton explains why the six mile stretch of the Mississippi River near Thebes, Illinois, is the only section of the navigation channel that is bedrock lined, with pinnacles of rock protruding into the shipping channel, making it difficult to dredge. The rest of the entire Mississippi shipping channel is loose unconsolidated sediment.
The ancient Mississippi River was re-aligned by earth quakes and glacial melt waters about 12,000 years ago, making the current narrow Mississippi River valley about 30 miles to the east of the location of the ancient Mississippi River valley. USACE has difficulty maintaining navigation in the shipping channel due to the bedrock lining and the fact that barges sometimes catch on the rock pinnacles.
During the drought of 2012 there were only a few feet of water in the Mississippi River except for the shipping channel, which is 9' deep and 300' wide. Giant excavators and a dragline were loaded on barges and moved out into the shipping channel to remove the rock during the drought of 2012 and early 2013. Shipping of agricultural grain and agricultural inputs on partially filled barges were move through the shipping channel at night, when there were only a few feet of water over the 9' deep and 300' wide channel. By February of 2013, the river rose and the excavators could not reach the remaining rock.
Approximately 70% of the rock was removed in phase 1. A second phase of the project has now started to remove the remaining bedrock pinnacles in the shipping channel and to re-align and widen the north end of the bedrock lined channel to make it easier to turn the long attached barges and keep them in the 300' wide navigation channel and avoid the underlying rock which is 9' closer to the water surface on the other sections of the 2000' wide Mississippi River bottom at Thebes, Illinois.
Feature article entitled "Dredging of the fractured bedrock-lined Mississippi River Channel at Thebes, Illinois" by Kenneth R. Olson and Lois Wright Morton and in the March-April 2014 issue of Journal of Soil and Water Conservation 2014;69 31A-35A: http://www.jswconline.org/content/69/2/31A.full.pdf.