CREVASSE!

Before Europeans arrived with their erroneous ideas about land and water management on a river delta, the Mississippi River was free to bleed out of its banks during high water through weak or low spots in its natural levee. This had the effect of nourishing the surrounding land with even more sediment and water. Sometimes the river developed a new route to the sea from one of these weak spots and created a new delta. Records indicate that once people began building artificial levees and constraining the river within its banks, the crest of its annual rise became higher and leaks became more explosive. These breaches are called crevasses.

In his Recent Geomorphic History of the Pontchartrain Basin, Roger Saucier researched historic crevasses from government reports, newspaper accounts and the evidence they left behind, like the tremendous quantity of sediment spewed out from the leak called a “splay.” Saucier examined some of the crevasses into the Pontchartrain system and found a couple of interesting things pertaining to the Isthmus of Manchac and the Manchac Greenway.

Bonnet Carre had at least six crevasses between 1849 and 1882. These left an extensive set of spays over what became the eastern Laplace area, reaching as far as the southwestern corner of Lake Pontchartrain. The amount of material it brought to this area was largely responsible for the beach ridge built up in the Frenier area that was settled by French and German cabbage farmers. 

The most dramatic crevasse came from the Nita Plantation near Romeville and Convent, LA in March of 1890 when about a third of the river broke out of the main channel into the surrounding swamp and on to Lake Maurepas. It filled the lake to overflowing with about eight feet of water then covered the Isthmus of Manchac about a month later, inundating and closing the railroad track from Frenier to just short of Ponchatoula. It lingered for two more months until rail service could resume. Strong east winds bottled this water up in Lake Pontchartrain, flooding the surrounding lowlands all the way up to the Metairie/Gentilly/Sauvage ridge.

After the disastrous Great Flood of 1927 was caused by too many crevasses, the federal government went to work preventing this from happening again by using three techniques: bigger and stronger levees, new water retention areas across the basin - from farm ponds to large reservoirs – and, especially to save south Louisiana, a pair of shunts or spillways. These are essentially fake, man-made crevasses that are shortcuts to the sea. They can be made to route away a good portion of the river’s flood into the Atchafalaya River at Morganza, LA and into Lake Pontchartrain at Bonnet Carre Bend, upriver from New Orleans.


Image from Recent Geomorphic History of the Pontchartrain Basin by Roger T. Saucier, LSU, Coastal Studies Series Number Nine.​