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<br />34 FLOODS OF SEPT. 1970 IN ARIZ., UTAH, COLO., AND N. MEX.
<br />
<br />however, there were no homes in the area in 1957, and the floodwater
<br />covered only meadows and woods. The contents of Vallecito Reservoir
<br />increased by about 11,000 acre-ft (13.6 hm') on September 6 and by 17,500
<br />acre-ft (21.6 hm') during September 5-7. The contents of the Navajo
<br />Reservoir on the San Juan River increased by 44,900. acre-ft (55.4 hm')
<br />during September 5-7. The contents of the reservoirs' are used mainly for
<br />irrigation. Because the flood of September 5-7 occurred immediately after
<br />the irrigation season when reservoir storage was low, the reservoirs were
<br />capable of storing most of the floodwater (U.S. Army Corps of Engineers,
<br />1970, p. 23).
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<br />The flood in the Animas River, which enters the San Juan River at
<br />Farmington, N. Mex., began late on September 5 near Silverton, Colo.
<br />Large flows in Mineral Creek-a tributary to the Animas River west of
<br />Silverton-and Bear Creek severed the waterline between Bear Creek and
<br />Silverton, and the water supply for Silverton was cut off. Dikes around the
<br />sewage ponds at Silverton were destroyed, which allowed sewage to flow
<br />into Mineral Creek. Between Silverton and Durango, Colo., U.S. Highway
<br />550 was closed at times owing to high water and mudslides. In the Animas
<br />River canyon downstream from Silverton more than 2 mi (3 km) of the
<br />Denver and Rio Grande Western narrow gage railroad was washed out (fig.
<br />13), and the railroad was closed for the rest of the season.
<br />
<br />Upstream from Durango, the Animas River flooded a broad valley, which
<br />acts as a natural reservoir for flows that overtop the channel. Some longtime
<br />residents stated that the area inundated was larger than that of the floods of
<br />1911 or 1927. The floods of>l911 and 1927 were oflong duration, the valley
<br />was flooded before the peaks arrived, and the peak flows passed through the
<br />valley with little attenuation. In 1970 the peak was extremely sharp and was
<br />reduced by storage in the valley. At Durango, the peak discharge was 25,000
<br />ft'l s (708 m'l s) in 1911,20,000 ft'l s (566 m' I s) in 1927, and 11,600 ft3 I s (328
<br />m'/s) in 1970.
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<br />FIGURE 13.- The Denver and Rio Grande Western narrow gage railroad in the Animas River
<br />canyon near Silverton after the flood of September 5-7, 1970. Photograph by Durango
<br />Herald.
<br />
<br />The flood in the lower part of the San Juan River basin mainly was the
<br />result oflarge flows from McElmo and Montezuma Creeks. Most of the flow
<br />from McElmo Creek was contributed by Yellowjacket Canyon. A graded
<br />road and a steel culvert were washed out in Yellowjacket Canyon, and one
<br />section of the culvert was washed three-eighths of ami (0.6 km) downstream.
<br />Two persons were drowned when a bridge approach washed out near Aneth,
<br />Utah, and their vehicle plunged into McElmo Creek. Montezuma Creek
<br />washed out about 600 ft (180 m) of Utah State Highway 262, which was built
<br />on fill across the natural channel. The flood tended to bypass a new cutoff
<br />channel, which was too small to accommodate the flow. The peak discharge
<br />of 40,500 ft'ls (1,150 m'/s) at the highway crossing (fig. 12, No. 56) was the
<br />highest since records began in 1959 and was 27 times greater than the
<br />previous maximum.
<br />
<br />On September 7, the flow in Chinle Wash near Mexican Water, Ariz. (fig.
<br />12, No. 60), reached a peak of 9,880 ft'/s (280 m'/s), which is the highest
<br />discharge in at least 20 years and is nearly three times the previous maximum
<br />for the period of record. Most of the flow originated in a small area south of
<br />Canyon de Chelly; small flows entered Chinle Wash from the Chuska
<br />Mountains.
<br />The failure of a levee along the San Juan River near Bluff, Utah, resulted
<br />in damage to fields and irrigation facilities. The peak discharge of 52,000
<br />ft'ls (1,470 m'ls) at the gaging station near Bluff, Utah (fig. 12, No. 61), was
<br />the highest since August 1929; however, the flood of October 1911 probably
<br />was the maximum at the station.
<br />
<br />FLOOD OF SEPTEMBER 12-14, 1970
<br />
<br />During the flood of September 12-14, peak discharges were generally
<br />lower than those during the flood of September 5-7; however, higher peaks
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