Levees and floodwalls are constructed to exclude flood waters from the protected area, up to a certain magnitude of flood. Unlike reservoirs and channel enlargements, the flood control effectiveness of a levee or floodwall will cease abruptly if a flood should overtop it. Interior runoff impeded by the structure may cause interior flooding if there are not proper provisions for interim storage behind it or discharge past the barrier. Potential effects outside a levee, upstream and downstream, are too complex and too site dependent to generalize otherwise, but generally the constriction of flow area caused by the structure will raise flood stages upstream. Within the levee reach, flood stages may be increased or decreased depending on whether the structure forms a hydraulically long or short constriction. A levee may reduce valley storage enough to cause the same impacts downstream as a channel. (USACE, Water Resources Policies and Authorities - Digest of Water Resources..., 1999, 13-4 and 5)