re: Non-Political Mississippi River Question

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If I understand the geography and hydrography of New Orleans and points south, historically (before dredging) the Mississippi basically became non-navigable for large vessels downstream of New Orleans as it spread into delta, making it necessary to use the Pontchartrain for ocean access. Is that correct?

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In a nutshell, but the merchant ships at the time were much smaller. Most could go into Lake Pontchatrain or up the river without much problem. New Orleans had access to both, and they even built a link between the two in the form of MRGO, which actually caused massive flooding to New Orleans and Chalmette during Katrina.

Once they put the levees up in the 20th century, the delta was more controlled in the main channel and the sediment was more constantly suspended in faster current (and carried farther out). So it didn't require as much dredging by the early 1900's. They still have to do it, but the river flow doesn't fan out over the marsh like it used to.

There is a plan in place that's actually fully funded to add a new outflow channel to put more sediment rich river water into the marsh south of New Orleans. In past events 20-30 years ago, they noticed flooding from the river on the east side way south of New orleans was actually causing the marsh to silt back up and create new land. So they are doing that again on the west side, only this time on purpose.

Louisiana is actually starting to get pretty innovative with rebuilding their coast line from the impact of erosion. I just wish it was moving much faster. The fishing industry is fighting it hard but they have ultimately lost this one.

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Goofball's answer was close, but not entirely correct.

"making it necessary to use the Pontchartrain for ocean access. Is that correct?"

Nope. As Goofball pointed out, merchant ships at the time were much smaller. But Lake Pontchartrain is a really a shallow pond (average depth of 10-12') and as such was rarely used for ocean-going sailing vessels.

"they even built a link between the two in the form of MRGO, which actually caused massive flooding to New Orleans and Chalmette during Katrina"

MRGO-- the Mississippi River Gulf Outlet-- does NOT connect Lake Pontchartrain and the Mississippi. It was completed in 1965 as a "short cut" for ocean-going vessels to access open waters of the Gulf.

What you are thinking of is the Inner Harbor Navigation Canal or IHNC, commonly called the Industrial Canal. Completed in 1923, it links the river to Lake Pontchartrain, but uses a set of locks to do so as the two waterways are at different elevations (river is higher; Lake Pontchartrain is essentially sea level). The Industrial Canal was built more for smaller cargo boats, barge traffic, etc. to allow access for boats bringing resources from "across the lake" or over in coastal Mississippi. It was never a "back door" for commercial ocean-going traffic.

The Industrial Canal was later incorporated into the Gulf Intracoastal Waterway (GIWW), a barge-oriented commercial waterway. When it was originally created in the 1930s, the GIWW had a route using the river, the IHNC, the Lake and the Rigolets to access Lake Borgne and the Mississippi Sound, but during World War II the GIWW was rerouted, with a newly excavated segment extending through the swamps west from the Rigolets joining the Industrial Canal at its approximate midway point between the river and the lake.

Schematic layout showing each:


Color representation. Blue line - 1923; green line - 1944; red line - 1965:


"Once they put the levees up in the 20th century, the delta was more controlled in the main channel and the sediment was more constantly suspended in faster current (and carried farther out)."

This wasn't a function of the levees, but a function of the jetties placed at the passes at the mouths of the river. In the late 1800s, silt sandbars often prevented ships from sailing in and out of the Mississippi River basin. Engineer James Eads postulated that jetties could solve the problem. Eads made a deal with Congress: he would build the jetties at his own expense, and if they maintained a 30-foot channel, Congress would then pay for the jetties.

In 1875, Eads built two parallel piers far out into the Gulf at the river's South Pass, which he predicted would narrow the river and increase its current. The concentrated current would then cut its own channel through the sandbar.

Eads’s jetties were completed in 1879 and maintained a 30-foot channel, eliminating the silt buildup, and opening up the mouth of the Mississippi once again to ships. New Orleans went from ninth largest port in the US to the second largest, just behind New York.