Bigger Pie Forum | The River Stands Still | Charles Grayson
Water flows downhill. The steeper the hill, the faster the flow. The slop from Natchez to the Gulf is a quarter that from the Ohio and Mississippi Rivers’ confluence to Greenville. From New Orleans to the Gulf, the drop is less than ¾ of an inch per river mile. Thirty feet of sediment at Old River Control Structure below Natchez creates a 30-foot-high sediment plug causing water backup and massive sand bars upriver to Vicksburg. During high flow, water is several feet higher at Natchez and Vicksburg than in 1988.
Natchez is the last major gauge on the Lower Mississippi River (MSR) to measure the river whole. A few miles south, the water is split with about 23% diverted through the Old River Control Structure (ORCS) into the Atchafalaya River Basin.
The remainder flows through Baton Rouge and New Orleans and is the basis for port facilities handling billions of dollars of freight including most U.S. agricultural exports. Decisions about managing the MSR are heavily influenced by demands of maintaining deep water ports, especially from Donaldsonville to Baton Rouge.
This article and one to follow will provide details about the MSR from Natchez to the Gulf of Mexico. In later articles, we’ll explore how this reach of the MSR is influencing the Mississippi up to the Yazoo River, Greenville and beyond.
Chart 1 shows the 2017 full year average surface water elevation of the MSR from New Madrid, MO to Venice, LA — 11 miles from joining the Gulf of Mexico. Below Natchez, part of the flow is diverted into ORCS and at Venice, part of the water flows out Grand Pass. Chart 1 shows a declining slope per river mile from Memphis south and a sharp decline in slope from Natchez south.
Chart 2 is an enlarged view of