Flood Hazards of Rainbow Valley, Maricopa county, aZ

The poor live on low ground waiting for the river to rise one night and sweep them out to sea. -Pablo Neruda

Strange though it may seem, valleys of the Sonoran Desert of central Arizona are subject to episodic flooding that can damage roads and buildings, destroy crops, and threaten lives. Nestled between the Sierra Estrella Mountains and the Maricopa Mountains southwest of Phoenix, Rainbow Valley is no exception. The valley floor, a checkerboard of agricultural fields, is drained by Waterman Wash that debouches into the Gila River to the north. From the valley axis, extensive piedmonts rise to the base of nearby mountains. 

AZGS’s newest map product, “Surficial geologic map and flood hazard assessment, Rainbow Valley, Maricopa County, Arizona”, displays the deposits of Rainbow Valley on three map sheets at 1:24,000 map-scale. These investigations were funded by the Flood Control District of Maricopa County (FCDMC).  The three sheets encompass portions of 12 USGS 7.5 minute quadrangles. 

The valley floor and adjacent piedmonts comprise a complex of fluvial sediments including channel and sheetflow deposits, terrace and bar deposits, alluvial fans, and sparse eolian silt and sand. The AZGS mapping team subdivided sediments into geologic units on the basis of sediment size and character, geomorphic expression, and relative age. Mapped units range in age from modern, through the Holocene or Late Pleistocene, to early Quaternary or late Tertiary.

The accompanying 11-page report includes a general Flood Hazard Assessment that groups geologic map units into high, intermediate, or low flood hazard areas. Within flood area categories, e.g., Flood Hazard High, units are arranged in order of decreasing frequency of flooding. For example, within the Flood Hazard High zone, unit Qycr – Modern channels of Waterman Wash – has the highest frequency of flooding of the seven geologic units; Qyaf – Potentially active alluvial fans, has the lowest chance of flooding. The three flood hazard areas encompass more than 250 sq. miles of Rainbow Valley and environs: Flood Hazard High (76 sq. miles), Intermediate (130 sq. miles), and Low (48 sq. miles). The purpose of this work is to provide a rough assessment of flood hazard areas; the FCDMC will use this information to guide future, more detailed flood hazard assessments before development occurs in Rainbow Valley.






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