Corps-Created Sill Gives NOLA Fresh Water

Every late summer/early fall, Mississippi River levels fall.  Less snowmelt and (this year especially, given the big drought) lead to a battle between the ocean and the river.  Saltwater, which is more dense than freshwater, crawls upriver along the riverbed.  To ensure New Orleans, Belle Chasse, Chalmette, etc. have potable water, the Corps of Engineers constructs a sill.  Just north of Myrtle Grove, dredging creates a sill that is only a few feet tall and is invisible to the casual observer (it never pokes above the water).  Despite that, it acts as a dam and ensures that the salinity of the freshwater intakes in the municipalities never exceeds acceptable levels.  It's not 100% effective; some saltwater does seep past, but it's never a problem for municipalities and industrial users (like boiler make-up water) can rent reverse osmosis units to keep it to acceptable levels for their use.  

The sill may be invisible to the naked eye, but here's a sonar image of the sill (from Meselhe 2005):

Just another part of the civil engineering that keeps the world humming every day.

UPDATE: Mike Schleifstein Writeup