A flood is a high water level that overflows the natural (or artificial) banks or levees along any portion of a stream. A flood plain is the region adjacent to a river that is frequently flooded. Floods are a natural part of a riverine environment, and occasional floods enrich soils. For example, the Nile river delta in Egypt experiences flooding nearly every year. This flooding keeps the land around the Nile fertile.
The fertility of frequently flooded land is what draws people to cluster near rivers. Worldwide, many people live within the 50 or 100 year flood plain of rivers. Flood plains are rated statistically based on how often they flood. For example, a 50 year flood plain floods, on average, every 50 years. People who live near rivers often build levees or artificially high banks around the river to contain it and try to avoid periodic flooding events. However, while levees keep one area from flooding, they heighten the flood risk for areas further down stream by constraining the water and 'pushing' more of it downstream.
K. Allison Lenkeit-Meezan (Foothill College)