<P> Because of the low terrain of the New Orleans area, a complex system of levees, canals, and pumps are required to reduce the risk of flooding . </P> <P> The natural levees along the Mississippi River were a result of soil deposits left from the river's annual floods . The site chosen for New Orleans had many advantages . Because it sits where distance between the river and Lake Pontchartrain is shortest, Louisiana Indians had long used the area as a depot and market for goods carried between the two waterways . The narrow strip of land also aided rapid troop movements, and the river's crescent shape slowed ships approaching from downriver and exposed them to gunfire, however flooding was always a hazard . </P> <P> The first artificial levees and canals were built in early colonial times . They were erected to protect New Orleans against routine flooding from the Mississippi River . The "back of town" away from the river originally drained down into the swamps running toward Lake Pontchartrain . Flooding from the lake side was rare and less severe as most of the old town had been built on high ground along the riverfront . </P> <P> As the city grew, demand for more land encouraged expansion into lower areas more prone to periodic flooding . For most of the 19th century most residential buildings were raised up at least a foot above street level (often several feet), since periodic flooding of the streets was a certainty at the time . </P>

When were the levees in new orleans built