A saltwater wedge is a layer of dense saltwater that moves upstream in a river, such as the Mississippi River, due to the difference in density between saltwater and freshwater. The leading edge, or "toe," of the saltwater wedge is well-defined, but some mixing occurs along the freshwater-saltwater interface, and mixed flows eventually reach the surface. The saltwater wedge is a problem for communities that rely on the river for their water supply because it brings unsafe levels of salinity to the water supply, leading to concerns about community health and pipe corrosion. The movement of the wedge is less responsive to increases in flows than to decreases, and when freshwater flows increase or decrease, the saltwater wedge retreats downstream or advances upstream, respectively. The U.S. Army Corps of Engineers built temporary underwater dams in the Mississippi River to block the progress of saltwater wedges that have formed several times over the last century. The Corps is also building an underwater levee of sand to stop the current saltwater wedge as it heads upstream.