When the Corps of Engineers started systematically leveeing the river in the 19th century, it cut off the region's main source of silt, the raw material of delta-building in the Mississippi Delta and the Wetlands of Louisiana. The weight of large buildings and infrastructure and the leaching of water, oil and gas from beneath the surface across the region have also contributed to the problem. Following the great floods of 1927, the Mississippi River was surrounded by a series of levees meant to protect the city from such floods. In 1965, New Orleans was hit by Hurricane Betsy, which caused tremendous flooding in the New Orleans area. The federal government began a levee-building program to protect New Orleans from a Category 3 hurricane (the same strength as Betsy). These series of levees were completed in recent years before Hurricane Katrina.