flood plains of major rivers like the tigris and euphrates in ancient mesopotamia, the yangtze and huang ho in china, and the nile in egypt. flood plains are good places to grow crops because as each flood inundates the plain, it carries with it a muddy sediment rich in organic matter and nutrients. the sediment is deposited in flat layers atop the flood plain and is naturally irrigated by the flood waters. but life on the flood plain is a double-edged sword. the agricultural benefits of those periodic floods are offset by damage to homes and cities, and, in some cases, to the people who inhabit them. the edges of flood plains are marked by levees- ridges of sediment left atop river banks by floods. once formed, levees serve as natural barriers confining rivers during periods of ordinary flow. they may even protect low-lying areas from flooding if the level of a river isn't too high. for this reason, artificial levees designed to contain a river during flood stages are often built. but artificial levees can themselves create problems. by confining the river to a narrow channel, levees ac