The sediments that underlie the Isthmus of Manchac were likely deposited by a northward distributary of the Mississippi River delta-front as it migrated through Louisiana’s River Parishes some 4,000 years ago. The two lakes formed on either side of this landform continue to subside and expand.
Centuries of Indian occupation and early European settlement left few scars on the land. It wasn’t until the 1850s that ambitious American entrepreneurs build a railroad through the swampy isthmus, cutting a swath of trees and installing a rail-bed, drainage and trestles. People could now see these once inaccessible swamps from the comfort of their railcars.
When the automobile became popular in the 1920s, the Hammond Highway was built near the railroad by digging a canal to build-up a roadbed. Then in the early 1960s, a more modern highway, US 51, was built alongside by actually filling in the Hammond Highway ditch and digging yet another ditch for the new roadbed. Additionally, tons of dredged clamshell from the lakes were used to shore up the base and sides of the road that we are now calling the Manchac Greenway.
Finally, in the 1970s, Interstate 55 was completed next door. At this time, the construction technique for building an elevated concrete highway through Louisiana’s swamplands required digging a massive canal in order to barge-in the cranes and other heavy equipment needed for driving-in the bridge’s legs and setting its decks. The dredge spoil from this “borrow” canal was left between old US 51 and the interstate. Bridges through swamp are now built using an “on-end” or top-down method that doesn’t require a canal that can damage the environment.
These projects and their maintenance have changed the hydrology and ecology of the isthmus.
Hydrologically, the wide I-55 borrow canal running the length of the isthmus makes the wetlands on either side much more accessible, and susceptible, to outside influences like salt water and new species of plants and animals. Local drainage is greatly influenced by cross-ditches.
The two old road beds and the dredge-spoil bank along the I-55 Canal are elevated, aereated earthen structures – levees. Add a blanket of rooted vegetation and rock armoring on the lakeshore and the entire Isthmus works as a “Coastal Line of Defense.”
Ecologically, the Greenway is radically different from the surrounding swamps and marshes and supports plant life that doesn’t normally grow here. Ironically, trees like Hackberry, Elm, Oak, Swamp Maple and Black Willow are much the same species found on old, meandering natural Mississippi River levees and shoreline Cheniers (beach ridges) across south Louisiana’s coastal wetlands.
Even the surrounding “natural” waterways and wetlands on either side of the Greenway’s transportation corridor are not the same as when the Europeans found them three centuries ago. There has been wholesale, industrial logging of the original primeval cypress swamp, some of which did not regrow because Mississippi River levees cut off nourishing spring overflows. Saltwater intrusion from storms and drought, canal digging and waterway dredging have helped convert much of what was originally cypress forest into marsh, particularly in the areas closer to the Manchac passes. Only the swamps toward the northern and southern ends of the isthmus have managed to regrow and regain much of their original ecology and appearance. Additionally, the human propensity to shift plant species between the continents has left us with a whole host of invasive plants, some clogging up the aforementioned waterways.