Bigger Pie Forum | A Deep Dive Into the Lower Mississippi River | Charles Grayson
Much of the Mississippi between Natchez and Venice, LA is below the surface of the Gulf of Mexico. A large portion of the river from Donaldsonville to Venice is below the BOTTOM of the entrance leading to the Gulf of Mexico (GoM). Chart 1 shows the elevations of the Gulf surface and the bottom in the reach of Venice to Heads of Passes where the Mississippi River (MSR) begins to merge into the Gulf.
The River’s surface elevations are taken from locations’ annual average gauge readings for 2017 which are higher than the past 70 years’ average, but normal for the 1990 to 2019 period. More detail is available in Table 1 at the end of BPF’s article – “The River Stands Still”.
The line depicting the MSR bottom approximates the average depth of significant portions of the river at each location. If a location has only a small deep portion but the majority of the region was 30 ft. higher, the higher average was used. The depths shown for about 50 river miles around New Orleans greatly understate the actual depth of the MSR bottom, which is -200 ft. in some places.
Note the bump in the river bottom below Natchez around mile 300. The bump begins just below Old River Control Station (ORCS) where 23% of the MSR waters are diverted to the Atchafalaya River Basin. This results in slower MRS flow just below ORCS and greatly increased sedimentation. A massive 20 to 25-mile long sand plug has developed. (See BPF’s article with video: Dr. Y. Jun Xu’s “Not a Matter of If, But a Matter of When” MSR in Danger of Being Captured by the Atchafalaya River.) From ORCS to the sharp river bend rounding at Donaldsonville –