Furthermore, the region's natural defenses, the surrounding marshland and the barrier islands, have been dwindling in recent years.[7] Much of the land was undeveloped swamp on the lake side, and only small levees were constructed in the 19th century. A much larger project to build up levees along the lake and extend the shoreline out by dredging began in 1927. As the city grew, there was increased pressure to urbanize lower areas, and, as a result, a large system of canals and pumps was constructed to drain the city. Drainage of the formerly swampy ground allowed more room for the city to expand, but also resulted in subsidence of the local soil.
Outside of the city, the Mississippi River's natural deposition of suspended sediment built up the river's delta marshlands during periodic flooding episodes. However, the lower Mississippi was later restricted to channels for the benefit of shipping, which interrupted the process that continued to build the Mississippi Delta and prevented its erosion. As the swampy lands of Southern Louisiana shrank, the land began to sink. Entire barrier islands disappeared during periodic storms as the land of the vast delta slowly settled without river silt to replenish the wetlands. Approximately one-third of the land subsidence has been attributed to the large number of canals through the delta. Barge traffic and tides erode the earth around the edge of the canals, and salty Gulf water seeps in along them, slowly salinating the ground and killing the vegetation that the land previously depended on to anchor it.[2]
