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<br />7
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<br />intense thunderstorms over small basins during the summer. Late
<br />season (July-October) tropical storms have occurred over large
<br />portions of the basin.
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<br />1.4 HISTORIC FLOODS IN THE COLORADO RIVER BASIN
<br />
<br />The largest recorded flood is that of July 1884, whi.ch was estimated
<br />to have had a peak flow of about 300,000 ft3/s on July 7 or July 8,
<br />in the Black Canyon of the Colorado, site of Hoover Dam. Estimates
<br />of this peak flow have ranged between 250,000 and 350,000 ft3/s.
<br />The 1884 flood had three distinct peaks, the maximun\ being around
<br />300,000 ft3/s. Many spring floods in the lower Colorado mainstem
<br />tend toward a generally broad shaped hydrograph. The 1884 flood
<br />estimate was made by the Bureau of Reclamation and the U.S.
<br />Geological Survey (USGS) and was based on gage height observations
<br />at Grand Junction, Colorado, and Yuma, Arizona: flood observations
<br />at Lees Ferry; and high water marks in the Black Canyon. The volume
<br />of the 1884 flood was estimated to be about 30,000,000 acre-feet for
<br />a five month period. Some evidence exists of a flood prior to that
<br />time, about 1862, that may have been greater, but estimates of its
<br />volume have not been made.
<br />
<br />On June 18-19, 1921, a flood peak of 220,000 ft3/s was observed
<br />at Lees Ferry. Other major floods of record with flows of over
<br />110,000 ft3/s occurred in the vicinity of Hoover and Glen Canyon
<br />Dams in 1880, 1885, 1886, 1901, 1905, 1906, 1907, 1909 (June and
<br />September), 1912, 1914, 1917, 1920, 1922, 1927 (July' and September),
<br />1928, 1929, 1941, 1952, and 1957. The flow into Hoover Dam would
<br />have exceeded 100,000 ft3/s in 1983 and 1984 if the upstream
<br />reservoirs had not been in place. Between 1878 and 1976 the flow
<br />has exceeded 100,000 ft3/s 31 times and has exceeded 200,000 ft3/s 3
<br />times, It has exceeded 50,000 ft3/s many, many times,
<br />
<br />Notable floods on the Colorado River and its tributaries were
<br />compiled from USGS Water Supply Paper 918 and other USGS water
<br />supply papers. These data were collected at USGS streamflow
<br />recording stations and are shown in table 1.2.
<br />
<br />Since Hoover Dam was completed in 1935, the largest floods occurred
<br />in 1941, 1952, 1957, and 1983. Maximum reservoir inflows were
<br />119,200, 122,000, 124,000, and 94,600 ft3/s, respectively. Maximum
<br />reservoir releases were 35,500, 30,900, 18,400, and 50,800 ft3/s for
<br />each of these years.
<br />
<br />Since Glen Canyon Dam was completed in 1963, the largest reservoir
<br />inflows occurred in 1983 and 1984. The maximum inflow to Lake
<br />Powell was 116,000 ft3/s in 1983 and 125,600 ft3/s in 1984.
<br />Reservoir releases were limited to a maximum of 97,300 and
<br />44,600 ft3/s in 1983 and 1984, respectively.
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