FLOOD05245

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Floods of September, 1970 in Arizona, Utah, Colorado, and New Mexico
• Date: 1/1/1978
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

STORMS AND WEATHER CONDITIONS

3

Parts of Arizona and Colorado were declared flood disaster areas and
were eligible for Federal aid. Flood damage was reported to be about $8.4
million in Arizona, and in the Four Corners area the damage from both
storms was about $2.9 million. Most of the flooding occurred in rural areas
and on national forest land; flood damage was minimized in downstream
areas where the reservoirs impounded most of the floodwater.
The purpose of this report is to describe the storms that produced the
floods, to give a factual account of the floods, to compare the floods with
past floods, and to document some of the channel modifications caused by
the floods. Although the report deals mainly with the flood of September
5-7, 1970, the flood of September 12-14 in the Four Corners area also is
discussed.

4 FLOODS OF SEPT. 1970 IN ARIZ., UTAH, COLO., AND N. MEX.

deep surges of tropical air into the state from the Gulf of California and the
Pacific Ocean. These storms, which occur most frequently in late August and
September, usually originate as tropical hurricanes off the west coast of
Mexico." The record rainfall of September 5, 1970, was the result of the
collision of a mass of moist tropical air from the Pacific Ocean with a cold
front from the northwest.
The flood-producing rains of September 3-6, 1970, were preceded by
near-normal rainfall in August, although during the last week many
climatological stations recorded no rain, and others recorded only minor
amounts. In most of Arizona the first two days of September were dry, but in
the Four Corners area light showers were common.

REPORTING OF DATA

The U.S. Geological Survey has adopted the policy of reporting data in
metric units in combination with English units as a first step in what
eventually will be total conversion to the metric system. For this report,
metric units are given in parentheses following English units in the text, and
English and metric units are shown in the illustrations, The data in the tables
are given in English units only.

STORM OF SEPTEMBER 3-6, 1970

METEOROLOGICAL SETTING

The following meteorological description of the storm of September 3-6 is
summarized mainly from the work of A. L. Zimmerman (U.S. National
Oceanic and Atmospheric Administration, 1971).
On September 3, tropical storm Norma was about 300 mi (500 km) west of
the southern tip of Baja California and was pushing moist tropical air
northward into southern Arizona while an intense cold front that extended
from northwestern Montana across Idaho and Oregon to northern
California was moving southward. Thunderstorms and heavy rain from the
tropical airmass began on the afternoon of September 3 in southeastern
Arizona. The moist tropical air continued to spread northward, and, by the
morning of September 4, clouds covered all Arizona and the Four Corners
area. The ITOS (bpproved Tiros Operational Satellite) I photograph taken
at 1453 hours, m.s,t. (mountain standard time), on September 4 shows that
the cloud cover had moved eastward and that the circulation around tropical
storm Norma continued to feed moist tropical air into the areas (fig. 2); the
clouds over northern Nevada are associated with the cold front approaching
from the northwest.
Rainfall was general over Arizona and the Four Corners area on
September 4 but was especially intense along the east side of the Baboquivari
Mountains in southern Arizona. By early morning of September 5, the eold
front extended diagonally across central Utah to the southern tip of Nevada
and across southern California. As the cold front approached from the
northwest, strong southerly winds developed over Arizona. The upward
flow of air ahead of the cold front combined with the strong orographic
uplift of moisture-laden tropical air caused extremely intense rain over
central Arizona and the Four Corners area. The intense rain began to fall
early on September 5 and continued throughout most ofthe day as thefront
moved steadily across Arizona and southern Utah. Rainfall activity was
about at its peak over central Arizona at 1550 hours, m.s.t., on September 5
(fig. 3). The rain had stopped by late evening in most of the central

.,

ACKNOWLEDGMENTS

The authors wish to acknowledge the assistance given by personnel of
several agencies in the preparation of this report. D. B. Thorud and P. F.
Ffolliott of the University of Arizona furnished flood-damage data, and A.
L. Zimmerman of the U.S. National Weather Service, formerly the U.S.
Weather Bureau, furnished rainfall and meteorological data. The Rocky
Mountain Forest and Range Experiment Stations ofthe U.S. Forest Service
at Tempe and Flagstaff furnished rainfall apd runoff data for small
watersheds in central Arizona. Many residents in the storm-affected areas
maintain nonrecording rain gages, and their data were obtained by the
authors and by th~ National Weather Service during a "bucket survey" made
immediately after the storm. Streamflow data were 0 btained from previous-
ly published reports and from the files of the U.S. Geological Survey offices
in Arizona, Utah, Colorado, and New Mexico.

STORMS AND WEATHER CONDITIONS

Summer rains in the flood areas generally are the result of the movement
of warm moist airmasses from the Gulf of Mexico. In late summer, however,
tropical storms from the Pacific Ocean occasionally bring large amounts of
precipitation into the areas, particularly those in Arizona. In a description of
the climate of Arizona, Green and Sellers (1964, p. 16) stated: "In fact most
of the record summer rains in the past century have been associated with