Saint Louis is protected from the Atlantic by the Langue de Barbarie, a long sand bank that extends from Mauritania along the coast of Senegal. The Langue is formed by the sediment from the Senegal river and allows the wetlands to flourish and supports a huge bird population and also provides freshwater for wells and agriculture. Unfortunately the Langue de Barbarie also causes flooding in the rainy season as the high river water has difficulty flowing to the sea as there is no direct route. In 2003 someone had the bright idea to cut a channel through the Langue in order to allow the water to escape and try to avoid flooding. Initially the channel was only 3 meters wide, but quickly expanded to over a kilometer wide.
Fishermen are happy because they now have a shortcut to get to the ocean instead of having to travel the entire length of the Langue. But now some of the consequences predicted by the environmentalists and scientists are starting to come true- the wells are becoming salty and agriculture is decreasing as the soil is becoming more salty as well. The wetlands are decreasing as well but the worst part is that the giant hole is starting to move south, threatening the expensive resorts located at the end of the Langue of the Barbarie. Scientist say that holes in the Langue are a natural phenomenon that take place every 50 years of so and they have previously measured the rate of speed of the hole as it moves south so they say this was to be expected (and part of their warning not to open the channel in the Langue). As this is a man-made hole this time they are glad to see that as the hole moves south the river is depositing new material at the north end of the hole.
More info about the Langue de Barbarie
-Wikipedia (see the maps): http://en.wikipedia.org/wiki/Langue_de_Barbarie
-Journal Article on Environmental Monitoring of the Langue de Barbarie Sand Spit and the Senegal River Estuary http://www.teledetection.net/upload/TELEDETECTION/pdf/20080404171636.pdf