Halfway between Pass Manchac and Laplace, the Isthmus of Manchac is at its lowest and narrowest between Lakes Pontchartrain and Maurepas. If you travel on the elevated Interstate-55 through this area on a winter day you can catch glimpses of either lake through the trees and the lower elevations here have allowed frequent flooding on the US 51/Manchac Greenway when lake levels are high. 

On this part of the isthmus you’ll also see lovely cypress-lined sloughs winding away from the road into the swamp. If you look at a map you’ll see these “finger lakes” are part of a cluster of narrow, east-west trending channels only found in this region of the isthmus. So, what and why are they?

There is no evidence that these channels are related to either the nearby Mississippi, Amite, Tickfaw or Blind Rivers. Neither can they be related to the construction of the highways because they show up on an 1892 map, well before the first road was built, nor are they likely to be related to the construction of the nearby railroad in the 1850s. Chances are these lakes have been here a lot longer.

Paul V. Heinrich with the Louisiana Geological Survey at LSU in Baton Rouge confirms these channels were likely scoured by water surging across this part of the isthmus when one lake was dramatically higher than the other. This might have happened several times over the 2,500 years these two lakes have been swapping water to and fro because of:

– Heavy rains in large upland drainage basins could surge into and overwhelm either lake, similar to what happened in the Amite River Basin into Lake Maurepas in 2016. 

– A Mississippi River crevasse could surge overland into these lakes and raise them to overflowing levels to cross the isthmus. For example, this happened at the Nita Plantation near Romeville and Convent, LA in March of 1890 when it is estimated about a third of the huge river broke away from its main channel and went through the surrounding swamp into Lake Maurepas. The lake filled to overflowing with an additional eight feet of water and covered the isthmus for about a month.

–  A tropical storm surge of sufficient intensity and timing from outside of the basin could cause a great imbalance between the two lakes. 

In either case, the normal route water takes to bring the two lakes into equilibrium is via the historically narrower Pass Manchac and the even narrower North Pass. If these became overwhelmed and unable to quickly covey this water, the water might, instead,  flow overland across the isthmus and help gouge out the finger lake channels here.

One of the dead-end “finger lakes” (Jasmin/Shell Bank Bayou).