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area for the third time in 5 years. The April 9 peak discharge had a frequency greater than 200 years. Of further interest at this location, only one flood greater than a 25-year event had been experienced during the first 78 years of record and that was in the first year of recorded events. During the last 5 years of record, three events have exceeded the 25-year frequency with the 1983 flood exceeding the 200-year event.

Again in late April and early May, the middle and upper Mississippi River basin experienced extensive flooding. Rainfall from this storm combined with snowmelt runoff to produce the fifth largest peak stage of record on the lower Mississippi River at Baton Rouge, Louisiana. The late May peak exceeded that of spring 1973, a flood that ended a 23-year period of relative tranquility. The 1983 discharge of about 1.5 million cubic feet per second was slightly greater than a 25-year frequency.

May 19 to 22 brought more suffering to the already weary people of central and northeastern Mississippi. The Pearl River

and the Tennessee Tombigbee River system were the hardest hit. More streams experienced record floods with frequencies in excess of 100 years.

For the 6-month period December 1, 1982, through May 31, 1983, the cumulative total rainfall exceeded 60 inches, or roughly 3 times the seasonal normal, and also exceeded the normal annual total rainfall. During the same period, the Pearl River at Carthage, Mississippi, experienced 51 inches of runoff, or roughly 85 percent of the known rainfall-produced streamflow.

At Jackson, Mississippi, flood stage was exceeded for 98 days from December 1, 1982, through May 31, 1983. Each month experienced significant flooding: December, 23 days over flood stage; January, 13 days; February, 21 days; March, 11 days; April, 18 days; and May, 12 days. In some locations, residents have had to evacuate their homes as much as 50 percent of the time.

With the summer thunderstorm season just beginning, heavy rainfall from July 2 to 4, 1983, in a band 50 miles wide from DeKalb, Illinois, to Valparaiso, Indiana,

Staff gage upstream near left abutment of Military Road bridge crossing French Branch Creek, a tributary of the Pearl River, near Slide/1, Louisiana, April 11, 1983. (Photograph by Verne Schneider, Water Resources Division, U.S. Geological Survey.)

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caused flooding with a frequency in excess of 100 years on some streams in northern Illinois. The 24-hour rainfall generally exceeded 5 inches, with a maximum of about 7 inches.

Virgin Islands

More than a foot of rain fell in a 12-hour period beginning late April 17, 1983, on St. Thomas and St. Johns. Torrents of water, mud, and debris, along with fallen rocks, trees, and utility poles, made most island roads impassable. The total cumulative rainfall from the fierce thunderstorm dumped more than 14 inches of rain on St. Thomas. Maximum reported rainfall was in excess of 18 inches on an island whose average annual rainfall is just 20 inches.

Extensive flooding was recorded in several areas throughout both islands. In Charlotte Amalie, the main city on St. Thomas and capital of the Virgin Islands, the Harry S Truman Airport had as much as 2 feet of water throughout the runway system and remained closed for 2 days. On both St. Thomas and St. Johns, urban areas were affected by as much as 3 feet of water. Throughout the islands, flooding exceeded previously known maximums with frequencies estimated to exceed 100-year events.

California—Coast and Western Slopes

California was plagued with severe storms from the Pacific during December 1982 through March 1983, and, in most of coastal California, the cumulative rainfall through the beginning of spring was approximately double the seasonal normal. In many parts of the State, the cumulative total rainfall equaled or exceeded 100-year frequency. The principal damaging activity associated with the rainstorms was the reactivation of deep-seated landslides as the rains infiltrated to raise the groundwater levels. Because infiltration in many areas requires days, weeks, or even months to raise local ground-water levels to maximum, the spring runoff from snowmelt triggered massive mud and debris flows and land and rock slides.

Localized floods, most often in urban areas, occurred with each storm. Flood peaks in California, although not generally exceeding many previous long-term maximums, were significant in comparison to

previous floods. On the Cache Creek near Yolo, the historic maximum of 41,400 cubic feet per second occurred on February 25, 1958. The January 27, 1983, peak discharge was 33,000 cubic feet per second; however, since the 1958 flood. Cache Creek has been partially controlled by the Indian Valley Reservoir, which was completed in 1974. The March 2, 1983, peak discharge on the Sacramento River at Butte City had a frequency of about 35 years. Clear Lake at Lakeport is the largest freshwater lake in the State. The lake level is regulated by gates on a dam at the outlet, and water is released down the natural channel on Cache Creek. The previous known maximum level for Clear Lake since 1874, which was 11.12 feet, occurred on January 28, 1914, before the dam was completed in 1915. The March 3, 1983, peak level was 11.16 feet.

The Great Basin

California, Nevada, and Utah experienced one of the most severe winters since the collection of climatic data began in the late 1880's. Many of the snowpack measuring sites set new records for depth and water content. Resulting runoff from this snow caused record peak discharges and cumulative-flow volumes at many sites. Temperatures for most of the winter and spring remained well below normal, with only a short break during early March 1983. Warm days and nights during this break, combined with rain on existing snow at altitudes below 6,000 feet, caused major flooding along the Humboldt River in northern Nevada and most small streams in eastern Nevada. The peak discharge on the Humboldt River at Elko exceeded the estimated magnitude of the 50-year flood.

In western Nevada, winterlike conditions with heavy snowfall continued through mid-May. Then the weather changed dramatically, and daytime temperatures rose into the 80's. Water saturated the steep slopes of the eastern Sierra Nevada causing numerous land-slope failures. On May 29, a 400-foot segment of the major road connecting Lake Tahoe to Carson City and Reno slid to the bottom of a 250-foot ravine, causing some minor damming but no major flooding on already swollen Glenbrook Creek. One day later, at noon on Memorial Day, a massive landslide on the flank of Slide Mountain displaced two

small lakes and sent a mud-and-debris flow roaring down Ophir Creek into Washoe Valley between Carson City and Reno. Preliminary estimates suggest that the flow traveled at speeds of about 20 miles per hour, with a peak-flow rate that may have been as great as 30,000 cubic feet per second.

Streamflow in the three major western Nevada rivers, the Truckee, Carson, and Walker, is breaking long-time cumulativeflow records. The yearly average discharge on the Truckee River at Reno is 657 cubic feet per second for 55 years of record. The projected average flow for the 1 983 water year is estimated at approximately 2,000 cubic feet per second. Pyramid Lake, the closed-basin terminus of the Truckee River, is expected to rise a total of 13 feet by the end of the water year. Already the lake has reached an altitude last seen in April 1950. The Carson River is projected to have a record average discharge of about 1,000 cubic feet per second for the 1983 water year. This will exceed the 42-year average by 254 percent. Walker Lake, the closed-basin terminus of

the Walker River, has already risen 7.5 feet since October 1, 1 982. Only two other similar rises have been recorded since 1928: 6 feet in 1938 and 7 feet in 1969. High flows in northern Nevada continued into late July.

In Utah, Great Salt Lake rose 5.2 feet from September 18, 1 982, to July 1, 1983, the greatest seasonal rise ever recorded. The previous largest known seasonal rise was 3.4 feet in 1 906-07. The rise in 1982-83 was caused by considerably above-average rainfall in 1982, above-average snowfall during winter 1982-83, and continued snowfall and unseasonably cool temperatures during spring 1 983 that prevented normal evaporation of water from the lake.

The level of Great Salt Lake on July 1, 1 983, was 4,205.0 feet above sea level. This is the highest level the lake has reached since 1924. The increase in area covered by the lake from September 1982 to July 1983 was 1 71,000 acres (267 square miles). This has created problems with the roads, railroads, wildfowlmanagement areas, recreational facilities,

Thirteenth South St. between West Temple and Second West, looking east, Salt Lake City. Utah, May 28, 1983. The conduit beneath Thirteenth South is unable to handle the runoff and spouts water into the street, sandbagged as a canal. (Photograph by Paul Blanchard, Water Resources Division, U.S Geological Survey.)

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and industrial installations that have encroached on the exposed lake shores.

The level of Utah Lake, by agreement of 1885 between the land owners around the lake in Utah Valley and the users of water from the lake in Salt Lake County, is to be controlled so that it does not rise above "compromise" level. In 1983, however, Utah Lake rose considerably above that level, peaking at 4.94 feet above "compromise" level June 23. Previous known maximum stages are 4.9 feet above "compromise" level in July 1884 and 6.42 feet above "compromise" level sometime in 1862.

In Utah, several streams exceeded previous known maximum floods, and recurrence intervals were greater than 50 and 100 years at many locations. The Sevier River near Lynndyl, for instance, experienced a peak discharge nearly double the previously known discharge since 1944 and more than double the magnitude of the 100-year flood.

In Salt Lake City, underground conduits have been designed to carry much of the streamflow. City Creek normally flows through a portion of these underground conduits. Red Butte, Emigration, and Parleys Creeks normally flow westward under the city in a conduit beneath 13th South Street. In late May and June 1983, the worst possible scenario of flooding had developed for the canyons that produce floodwaters for these streams. Conduits in the city became blocked with silt, gravel, and debris swept down from the channels and flood plains where it had accumulated during long periods without major floods. The conduits could not begin to handle all the water which overflowed into the streets above. The streets were sand

bagged and formed into canals, and floodwaters were channeled down 13th South and State Streets —thus, the emergence of two new rivers, the "State Street River"and the "13th South Street River."

Colorado River Basin

Larger than normal accumulation of snow occurred in the Rocky Mountains during winter and spring 1982-83. The U.S. Soil Conservation Service maintains a network of snow courses, some telemetered, to provide information on the rate of snow accumulation and equivalent water content. Preliminary data and information show that as of January 1, 1983, the equivalent water content of the snow cover in much of the Rockies was 121 percent of the long-term (1963-77) average. Lack of snow during January and February caused estimates of the snow water content to be decreased to about 90 percent of the long-term average on February 1 and March 1. Then, late winter and early spring snows caused estimates of the equivalent water content to be increased to 120 percent of the 1963 to 1977 average April 1, and, by May 1, estimates were 142 percent of the longterm average.

Cooler than normal temperatures in May caused snowmelt from only the lower elevations. This is illustrated by the fact that some stations that receive runoff from lower elevations experienced peak discharges about the end of May, and the frequencies were generally less than 10 years. However, a larger than normal snow cover still existed above 8,000 feet. Melting of the high elevation snow took place at a gradual rate until the last week

of June when daytime high temperatures in the mountains reached 70° to 80°F. and a frontal system moved through from June 23 to 25 bringing general rainfall in excess of 0.5 inch. This combination of warm temperatures and rainfall generated peak discharges at gaging stations that had a range in recurrence intervals from 5 to 200 years. The most significant high flows occurred on the Colorado, Gunnison, and White Rivers. An outstanding example occurred near Meeker, Colorado, on the White River where the discharge exceeded all previous maximum discharges that occurred in 78 years of station operation. The peak discharge frequency was estimated to be about 200 years.

Flooding in Arizona and California along the Colorado River was caused by releases from Lake Powell (Glen Canyon Dam) and Lake Mead (Hoover Dam). A June 6, 1983, forecast predicted inflow to Lake Powell for the months of April through July at 1 31 percent of normal runoff. Updated estimates of inflow to Lake Powell made June 28 called for 210 percent of normal runoff.

Inflow to Lake Powell increased from about 1 6,000 cubic feet per second to over 30,000 cubic feet per second in late April 1 983. In late May, the inflow increased from 36,000 to about 90,000 cubic feet per second. The peak inflow of about 126,000 cubic feet per second occurred in June. On July 10, estimated inflow was 69,000 cubic feet per second. Releases from Lake Powell increased steadily during June, reaching a maximum average daily discharge of 92,600 cubic feet per second.

Releases from Lake Mead were increased in January to meet storage

amounts in accordance with operation criteria. In late June, releases from Hoover Dam were being held at 40,000 cubic feet per second. However, flow over the spillway of Hoover Dam began on July 3, and the maximum release was 42,000 cubic feet per second on July 10. Flow over the spillway had occurred only one other time since the completion of the dam in 1 935 and that occasion was in 1 943 during controlled operations to test the newly completed system.

Summary

Above-normal precipitation prevailed in most of the United States in 1 982 and 1983, filling lakes and reservoirs, recharging many aquifers, and, when unusually excessive, causing floods. Meltwaterfrom extremely heavy snows in parts of the West caused floods and posed continuing threats of flooding in many areas in spring and summer 1983.

The intense flooding that affected much of the Mississippi and Missouri River basins, the Great Basin, and the Colorado River basin boosted streamflows on many streams to their highest in 55 years, possibly even during the 20th century. The frequency for floods in parts of at least 14 States exceeded that which can be expected to occur on the average of once in 100 years, with a few exceeding frequencies of once in more than 200 years. When all the data and information have been collected, analyses might very well show such widespread climatic and hydro logic events in one year to be the unique situation of modern man's experience.

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