The Williams Canal across De Soto Peninsula had been abandoned by Adm. Farragut and Brig. Gen. Williams in July 1862, but it had the potential to offer a route downriver that bypassed Vicksburg's guns. In late January 1863, Sherman's men, at the urging of Grant—who was advised by the navy that President Lincoln liked the idea—resumed digging. Sherman derisively called the work "Butler's Ditch"* (since it was Maj. Gen. Benjamin Butler who had sent Williams upriver to do the work), which was barely 6 feet wide by 6 feet deep. Grant, undoubtedly influenced by Lincoln's continuous inquiries as to the status of the canal, ordered Sherman to expand the canal to 60 feet wide and 7 feet deep and the effort became known as Grant's Canal. It was not properly engineered based upon the hydrology of the Mississippi River, however, and a sudden rise in the river broke through the dam at the head of the canal and flooded the area. The canal began to fill up with back water and sediment. In a desperate effort to rescue the project, two huge steam-driven dipper dredges, Hercules and Sampson, attempted to clear the channel, but the dredges were exposed to Confederate artillery fire from the bluffs at Vicksburg and driven away. By late March, work on the canal was abandoned. (Remnants of about 200 yards of Grant's Canal are maintained by the Vicksburg National Military Park in Louisiana).[6-Bearss, vol. I, pp. 436-50; VNMP article on Grant's Canal]
http://en.wikipedia.org/wiki/Vicksburg_Campaign

* Also called "Folly Creek"