Flooding Defined
Floods are the most common natural disasters in the United States. A flood is defined as the temporary inundation of water on normally dry land from the overflow of inland waters (such as lakes and rivers) or the rapid accumulation or runoff of surface waters from rainstorms.
Maricopa County, even with its desert climate, has the potential for major flooding. Floods in the County have caused damage claims totaling hundreds of millions of dollars. Some parts of the County have even received presidential disaster declarations.
How Floods Occur
Flooding in the County occurs when natural waterways such as creeks, rivers and washes cannot contain the flow of a large rainfall event. It is expected that waterways and adjacent land will flood during heavy storms. These areas are officially delineated as 100-year floodplains. In any given year, there is a one percent chance that a storm will happen which will flood the entire floodplain. In addition to 100-year floodplains, there are "ponding" areas where water collects in low spots, such as along the embankments of railroads, highways, irrigation canals and flood control channels.
Floods that create significant damage in a desert environment occur due to these conditions:
- widespread, high-density development for a large population
- location in a major drainage basin
- the frequency of severe storms with greater-than-normal precipitation
- inadequate drainage
Sources of Flooding in Maricopa County
Maricopa County, especially the Phoenix metropolitan area, is affected by all four conditions. The growth of the County's population over the years has produced agricultural development followed by significant urban development. In many cases, streets, roads, farms and subdivisions have been developed with little regard to the County's natural drainage basin and its river systems. Buildings, homes, and paved streets and parking lots have disrupted the natural flow of water and prevented absorption into the ground, creating inadequate drainage in some areas. On top of that, the County experiences two active precipitation seasons each year with a high frequency of severe storms.
Maricopa County has a natural system of washes, streams and rivers that can carry off and drain accumulated rainfall. The Phoenix metropolitan area, for example, is located within four major river systems. The Salt River flows in a westerly direction through the southern third of the metropolitan area. The Verde River on the eastern side of the region flows south to the Salt River. The Agua Fria River on the western side flows south to the Gila River. The Salt, Verde and Agua Fria rivers all feed into the Gila River, which is the primary watercourse in Maricopa County. A fifth major river, the Hassayampa, flows south through the communities of Wickenburg and Buckeye before entering the Gila River north of Gillespie Dam. In addition, the New River in northern Phoenix and a series of major washes provide drainage in the County.
The major river systems in Maricopa County drain an area of approximately 57,000 square miles that includes regions of New Mexico and Mexico. Rainstorms can fill rivers and washes outside the County, sending floodwaters through tributaries into the County's drainage system. In fact, some of the most damaging floods have occurred from winter storms outside the County.
There are two separate precipitation and flooding seasons in Arizona. The first occurs from November to March. Winter precipitation is greatest when the storm track enters the state from the west or southwest after picking up considerable moisture from the Pacific Ocean. These storms are generally more regional in nature and can affect one or more of the major river systems during the same period of time.
The second rainfall season, known as the "summer monsoon," occurs in July, August and September when the County experiences widespread thunderstorm activity associated with moist air from the south and southeast. These thunderstorms are extremely variable in intensity and location, and can produce heavy amounts of precipitation in short periods of time. The flooding that may result is localized and short-lived, however the damage can be severe.
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