Graphic: How a sill on the Mississippi riverbed slows salt water, allows ships to pass

When it was first constructed in July, the sill at the bottom of the Mississippi River north of Myrtle Grove in Plaquemines Parish gave the river a uniform depth of about 60 feet. The idea was to block heavier salt water from pushing further upstream as the river's volume of fresh water slackened. But the Mississippi's flow has continued to slow, and the "toe" of the salt water -- its leading edge, which hugs the river bottom -- has already passed the sill. Modifications begun by the Corps of Engineers...