Impacts of Climate Change and Land Use  on the Southwestern United States

Southwestern States Flood and Drought Summaries


This page is excerpted from Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., Compilers, National Water Summary 1988-89-- Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.

Floods and droughts have had major effects on Arizona's development. The most significant floods and droughts in Arizona are listed chronologically in Table 1. Records from streamflow-gaging stations were analyzed to determine recurrence interval and areal extent of the floods and droughts. Streamflow data are collected, stored, and reported by water year (a water year is the l 2-month period from October l through September 30 and is identified by the calendar year in which it ends). The gaging stations were selected to represent natural patterns of runoff for the entire State.

Table 1. Chronology of major and other memorable floods and droughts in Arizona, 1862- 1988

Flood or DroughtDateArea AffectedRecurrence Interval (in years)Remarks
FloodJan. 19-23, 1862Gila and Colorado RiversUnknownSevere at Yuma. Wet year in Verde and Bright Angel basins, but not in upper Salt.
FloodFeb. 18- 26, 1891Central Highlands25 to 100Phoenix and Yuma flooded. In Clifton, deaths, 18; damage, $1 million.
FloodNov. 27- 30, 1905San Francisco to Verde Rivers5 to 10 Several moderate to severe floods, particularly at Phoenix and along the lower Gila River.
FloodJan. 19- 22, 1916Central Highlands10 to 0Intense rain on melting snow produced large flows in central Arizona. Deaths, 4; damage, $300,000.
FloodAug. 21, 1921Phoenix (Cave Creek)UnknownSix inches of rain in two days flooded 4,000 acres and the State capitol building, Damage $240,000
FloodSept. 27- 29, 1926San Pedro River and Mexico>100Tropical storm. Peak flow 2 - 3 times larger than any other in 70 years. Damage, $450,000
Drought1932- 36Statewide10 to 20Effects differed among basins.
FloodMar. 14- 15, 1941Central Arizona5 to 40One of several storms that caused general runoff and filled reservoirs
Drought1942- 64Statewide>100Second most severe in 350 years, on the basis of tree-growth records.
FloodSept. 26- 28, 1962Brawley and Santa Rosa Washes>100Deaths, 1; damage, $3 million, mostly to agriculture near Casa Grande.
FloodDec. 22, 1965 to Jan. 2, 1966Verde, Salt, and Gila Rivers and Rillito Creek.10 to 50First large flow through Phoenix since reservoirs were built on Verde River (1939). Damage, $10 million.
FloodDec. 5- 7, 1966Grand Canyon to southwestern Utah.>100Mudflows and channel erosion damaged Indian ruins that had been undisturbed for 800 years.
FloodSept. 5- 7, 1970Tonto Creek to Hassayampa River.40 to 100Labor Day weekend floods in recreation areas. Reservoirs stored most runoff. Deaths, 23; damage, $8 million
FloodOct. 17- 21, 1972Upper Gila River10 to 40Tropical storm. Deaths, 8; damage, $10 million.
Drought1973- 77Statewide15 to 35Most severe in eastern Arizona.
FloodJuly 17, 1974Safford (Holyoke Wash)>100Thunderstorm produced flow of 1,740 cubic feet per second from 0.85 square mile.
FloodOct. 1977 to Feb. 1980Central and southeastern Arizona5 to 100Seven regional floods. Phoenix declared a disaster area three times. Deaths, 18; damage, $310 million.
FloodJuly 26, 1981Tucson (Tanque Verde Falls)less than 2Flash flood at recreation area on Sunday; deaths, 8. Two larger peak discharges in the same week were not noticed.
FloodJune 20 to Aug. 17, 1983Colorado River20 to 40Upper basin rain and snowmelt. First reservoir spill since Hoover Dam was built (1935). Damage, $80 million.
FloodOct. 1- 3, 1983Santa Cruz to San Francisco Rivers10 to >100Record floods on 18 streams; two peak discharges doubled 65-year-old records. Deaths, 8; damage, $226 million.


Figure 1. Areal Extent of Floods in Arizona. Click to view larger image.

Arizona has both local and regional floods. Local floods occur most frequently. Intense thunderstorms, typically from July to September, can produce peak flows of 2,000 ft3/s (cubic feet per second) in a basin of 1 mi2 (square mile). A flood peak can appear in a dry channel in a few minutes and disappear in a few hours; peak discharges commonly decrease downstream. Local floods occur primarily in basins of less than about 100 mi2 because the area affected by an individual thunderstorm is small. Cumulative damage from local floods can be substantial, even though a single flood rarely causes much damage. Local floods cause several drownings annually in Arizona. A local flood on Cave Creek on August 21, 1921, inundated the State capitol building in Phoenix (Table 1). The July 17, 1974, flood in Holyoke Wash near Safford was the largest recorded in Arizona from less than 1 mi2.

On Sunday, July 26, 1981, about 100 people were playing in the water at Tanque Verde Falls, a series of waterfalls in a small canyon about 15 mi east of Tucson (The Arizona Daily Star, Tucson, August 9, 1981). A small thunderstorm that could not be seen from the canyon caused the water level to rise 4 feet and the discharge to increase from 50 to about l,500 ft3/s in about 1 minute (Hjalmarson,1984). Eight people were killed. A gaging station 1.5 mi downstream recorded a peak discharge of 836 ft3/s, and the flow was unnoticed near Tucson, where the stream spreads into a wide sand channel . Peak discharges larger than that of July 26, 1981, had occurred at the gaging station in 18 of the previous 25 years. Even more striking was the fact that discharge rose to 5,000 ft3/s on the Saturday morning before the tragedy and 6,700 ft3/s on Thursday night afterward. These larger peaks were not noticed by the public and caused no damage because no one had been at risk on the canyon streambed when the peaks occurred.

Regional floods generally occur between September and March in drainage basins larger than 200 mi2. The areal extent and severity of five regional floods, as determined from streamflow data and other sources, are shown in Figure 1. These floods are caused by several days of intense regional rainfall, sometimes combined with snowmelt. Peak discharges commonly increase downstream as each small tributary basin adds its flow. Regional floods are most common in the Central Highlands province, where rainfall intensifies as moist air rises to cross the mountains. High flow that continues for several days can cause major erosion damage. During a flood, a river can move hundreds of feet laterally, especially at the outside of bends in the streambank. Buildings and irrigation works near the streambanks can be undercut and destroyed. The channel bottom can erode vertically 20 feet or more, although sediment deposited as the flood recedes commonly raises the channel bottom to near its original level. Bridge foundations and pipelines crossing a river can be destroyed.

During February 18-26, 1891, the Salt River at Phoenix had a peak discharge of 300,000 ft3/s. This was the largest recorded flood in Arizona history and was preceded by high streamflow in 1890. The river became 2-3 miles wide and extended 2 miles north of the channel in central Phoenix. The flood resulted from a series of frontal systems from the Pacific Ocean that caused extensive concurrent flooding in southern California. Although probably widespread in the Central Highlands. the extent of flooding is mapped only along major rivers because too few people lived in other areas to provide consistent reports of flooding.

Between October 1977 and February 1980, seven regional floods affected nearly all the people of Arizona. Five of the floods had recurrence intervals of at least 25 years in some part of the State. Many areas were affected by several of the floods, and Phoenix received Federal flood disaster assistance funds three times within 24 months. By March 1979. floods on the upper Salt and Verde Rivers had caused $230 million in damage and filled reservoirs upstream from Phoenix (Aldridge and Eychaner, 1984; Aldridge and Hales, 1984). Then during February 13-25, 1980, another flood occurred that had the greatest impact on the largest number of residents in Arizona history. Six storms during 9 days moved from the Pacific Ocean into southern California and Arizona. Daily rainfall quantities in Arizona were not extraordinary, but the total volume of runoff far exceeded available reservoir capacity. The peak discharge of the Salt River at Phoenix was 170,000 ft3/s, which was greater than any previous flow since 1905. The flood caused $80 million in damage (Chin and others, 1990).

The regional flood of October 1-3, 1983 (water year 1984), caused $226 million in damage in southeastern Arizona (Roeske and others, 1989). Moisture from Tropical Storm Octave, which dissipated off the west coast of Mexico, moved northeastward across Arizona for several days. Rainfall was most intense in a narrow band from south of Tucson to Clifton. Peak discharges on the Santa Cruz River at Tucson and on Aravaipa Creek were more than twice those observed in the previous 65 years. Recently installed soil-cement streambank reinforcement helped to limit damage in Tucson; nonetheless, several buildings were lost to erosion. Large areas northwest of Tucson were inundated as floodwaters from the Santa Cruz and Gila Rivers spread across flat fields. The flood was the largest on record at San Francisco River at Ciifton hut was only 1 of 12 floods that have inundated the town since it was established in a canyon on a narrow flood plain (Hjalmarson,1990). Flood-protection structures protect Clifton from small and medium floods, but almost the entire town can be damaged by any flood that overtops the floodwalls.


Figure 2. Areal Extent of Droughts in Arizona. Click to view larger image.

Arizona streams technically are under drought conditions 60-80 percent of the time, and individual droughts commonly last as long as 5 years. Drought is defined as an extended period of less than average streamflow. Arizona streams have short periods of large discharge and long periods of small discharge, which make "average streamflow" larger than the rates that water users ordinarily have available. Stream channels in the deserts of the Basin and Range Lowlands are ordinarily dry and contain no useful water. Arizona's Central Highlands receive more precipitation than the lowlands and maintain perennial streamflow in large basins (Brown and others, 1981). Reservoirs have been constructed on most major perennial streams in Arizona to hold and distribute floodwater or spring snowmelt for use in the desert. These reservoirs decrease the effects of short-term droughts, but because of the extensive development of agriculture, industry, and cities in the desert, a prolonged drought can affect a large population.

The areal extent and severity of three droughts (1932-36,1942-64, and 1973-77), as determined from 15 long-term gaging stations, are shown on the maps in figure 2.

The major 20th-century drought in Arizona, as determined from streamflow records covering 78 years, occurred from 1942 to 1964 (Fig. 2). The drought was interrupted temporarily at some gaging stations in 1949 and 1952. Reservoirs supplying the Phoenix area were replenished by high flows in 1952. At Rillito Creek near Tucson, the drought of 1942-64 decreased recharge to ground water rather than decreasing usable surface water. In southeastern Arizona, the drought ended in the middle to late 1950's, but a new drought began in the early 1960's.

Streamflow data indicate that the drought of 1942-64 had recurrence intervals of 20-60 years. However, the drought's duration is such a large part of the total length of record at many sites that the computed recurrence intervals may be unrealistically small. A better assessment of the frequency with which such a drought will recur is available from indirect evidence. Streamflow volumes have been estimated for as many as 350 years at three sites on the basis of measured rates of tree growth (Stockton, 1975; Smith and Stockton, 1981). Departures from the average of the estimated annual flows indicate that the 1942-64 drought was the second most severe on record at each site: its recurrence interval probably is greater than 100 years. The drought of 1801-23 was more severe at two sites, and the drought of 1721-41 was more severe at the third site.

Several shorter droughts are apparent in the streamflow records, most notably those of 1932-36 and 1973-77. During 1932-36, a drought having a recurrence interval of about 10-20 years affected most of the State. The drought of 1973-77 also was widespread; the recurrence interval for that drought ranged from about 15 to 35 years, depending on location. On the San Pedro River at Charleston, the 1973-77 period was the end of a longer drought that began in 1960.

Selected References

Aldridge, B.N., and Eychaner, J.H., 1984, Floods of October 1977 in southern Arizona and March 1978 in central Arizona: U.S. Geological Survey Water-Supply Paper 2223, 143p.

Aldridge, B.N., and Hales, T.A., 1984, Floods of November 1978 to March 1979 in Arizona and west-central New Mexico: U.S. Geological Survey Water-Supply Paper 2241, 149p.

Chin, E.H., et. al., 1990, Floods of February 1980 in southern California and central Arizona: U.S. Geological Survey Professional Paper 1494

Roeske, R.H., Garrett, J.M., and Eychaner,J.H., 1989, Floods of October 1983 in southeastern Arizona: U.S. Geological Survey Water Resources Investigations Report 85-4225-C, 77p.

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