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STORMS AND WEATHER CONDITIONS 9 TABLB l.-Comparis~n ofmaxim~m observational day precipitation during September 3-6. 1970, wuh that prevIously known at selected stations in Arizona (From U.S. Environmental Data Service, 1970a] Maximum Maximum SeptemberJ-<5 observational day Station Station observational day precipitalion established precipitation previously known ()T) Day Amount Year AmOUnI (in.) (in.) Bar T Bar Ranch ...................,..... 5 5.30 1955 3.96 1952 Bartlett Dam .................;.............. 6 4.50 1951 4.00 1939 Groom'-Cieek ............................... 5 4.25 1966 3.85 1942 Junipine ....................................... 5 5.28 1937 4.71 1935 Mummy Mountain ...................... 5 3.94 1966 2.29 1955 Payson 12NNE ........................... 5 4.29 1967 3.53 1950 Payson Ranger Station ................ 5 6.20 1959 4.37 1892 Payson ..............,..........,............... 5 5.36 1959 3.74 1948 Sasabe ....... ........ ........ ...... ....... ...... 4 4.36 1969 2.75 1959 Sedona Ranger Station "........."", 5 550 1958 2.69 1943 Sierra Ancha .........................._,.... 5 4.77 1951 4.58 1935 Tonto Creek Fish Hatchery ......................'........... 6 5.63 1957 4.30 1944 Workman Creek I' ...,.................. 5 10.99 1954 5.27 1941 lData ftom Kansieser, 1972. Mountains; 3 in: (75 mm) of rain fell in 3 hours at Payson and al Workman Creek I m the SIerra Ancha (U.S. Environmental Data Service 197Ib)' and more th~n 3 in. (?5 mm) fell in 4 hours at Sycamore Creek in ihe Maz~tzal Mountams. MaxImum 15-minute rainfall intensities of more than 2.5 in. (64 mm) per hour ?ccurred at several stations operated by the U.S. Forest ServlCe m the Sierra Ancha, the Mazatzal Mountains, and on the plateau southeast of Flagstaff (Thorud and Ffolliott, 1973, p. 5). . A comparison of the maximum 24-hour precipitation for September 3-6 wIth the 100~y~ar. 24-~our precipitation is given in table 2. The 100-year 2~-hour preclpltalIon IS the amount of precipitation in a 24-hour period that wIll be equaled or exceeded, dunng a long period of time, on the average of once every 1?0 years. The recurrence interval is a probability concept and does not mdlCate the amount of lIme between such precipitation events. In general the 100-year 24-hou~ preci~itation is 4.6 to 6.0in. (1I7to 152 mm) in the moun tams of central ArIzona; m a few small areas near the crests of the mountam~, h?wever, and along the Mogollon Rim the 100-year 24-hour precIpitatIOn IS 6.5 to 7.0 in. (165 to 178 mm). (See U.S. Weather Bureau 1967a.) From an analysis of the 30-year record at the Workman Creek i slation in the Sierra Ancha, the recurrence interval of the rainfall on September 5 is estimated as more than 500 years (Kangieser, .1972). In 10 FLOODS OF SEPT. 1970 IN ARIZ.. UTAH. COLO., AND N. MEX. addition to the areas where 24-hour precipitation is known to have exceeded the 100-year value, several areas in the Navajo and Hopi Indian Reser- vations in northeastern Arizona had large amounts of runoff, which indicate that the precipitation may have been in excess of the 100-year value. STORM OF SEPTEMBER 12.14, 1970 A second storm hit the Four Corners area September 12-14. Rain fell in southwestern Colorado and southeastern Utah as a cold front moved southward over Colorado bringing rain behind it. In the Four Corners area the amount of rain generally was less than during the previous storm, and most climatological stations reported less than 2.5 i~. (64 mm) during the period. The largest amount of rain measured during the storm was at the station I mi (1.6 km) east of Wolf Creek Pass in the San Juan Mountains near the divide between the San Juan River and the Rio Grande; 4.83 in. (123 mm) was reported for the 3-day storm, and the largest 24-hour precipitation was 2.84 in. (72 mm). In the Four Corners area the network of climatological stations is adequate to define only the general precipitation pattern (fig. 5). In several places, such as in the extreme northeast corner of Arizona and in the Indian Creek basin northwest of Monticello, Utah, runoffindicated that rainfall was greater than that shown in figure 5. DESCRIPTION OF THE FLOODS The storms of September 1970 in Arizona and the Four Corners area produced record peak discharges in many places and caused channel modifications along many of the streams (table 3). The floods of September 1970 are discussed under four geographical areas in this report: (I) southern Arizona, (2) central Arizona, (3) northeastern Arizona, and (4) Four Corners area (fig. I). The most severe floods occurred in the mountains of central Arizona and in the Four Corners area. SOUTHERN ARIZONA In southern Arizona flooding began on September 4, and the last thrust of the slorm caused additional flooding on September 6. The intense rain on the Baboquivari Mountains near Sasabe induced flooding in the Los Robles Wash basin on September 4. The main stream in the Los Robles Wash basin is called Altar Wash in the upper part of the basin, Brawley Wash in the middle part, and Los Robles Wash in the lower part. Large flows entered Altar Wash from tributaries on the west, but little flow came from those on the east. Generally, the flow was confined in the deep channel in the upper part of Altar Wash. The flood destroyed the gaging station on Altar Wash at the bridge on State Highway 286 (fig. 6, No. 78) and also destroyed the steel-truss bridge, which was constructed in 1915. . The flood in Brawley Wash inundated the gaging station at the bridge on State Highway 86 near Three Points (fig. 6, No. 79); the bridge was