Flooding

An underlying influence on many floods is the hydrologic cycle. The hydrologic cycle is the evaporation of water from the oceans into the atmosphere from which it falls as rain or snow on land. The water, then, runs off the land or is absorbed by it and, after some period of time, makes its way back to the oceans. Scientists have found that the total amount of water on Earth has not changed in three billion years; therefore, the hydrologic cycle is said to be constant. The same water has been filtered by soil and plant use and purified by temperature changes over many generations. Rivers and streams may feed water into the ground, or where springs persist, Flooding on the Salt River, Arizona. JLM Visuals. Reproduced by permission.
groundwater may supply water to streams (allowing them to flow even when there is a drought).

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place, year after year. If it occurred consistently in all locations, floods and droughts would not exist. Thus, some places on Earth experience more than average rainfall, while other places endure droughts. It is not surprising, then, that people living near rivers eventually endure floods.