Southwestern States Flood and Drought Summaries
From 20 to 30 floods of some significance occur somewhere in Colorado every year. Annual flood losses in Colorado have averaged four deaths and $ 14 million in property damage for the period 1896- 1976. In the past 20 years, nine major-disaster areas have been identified by Presidential declaration because of flooding in Colorado. Since Colorado became a State in 1876, floods have killed at least 350 people and caused cumulative flood losses of about $1.7 billion at present ( 1988) value (Colorado Water Conservation Board, 1985, p. vii).
Irrigation is the principal use of surface water in Colorado and in 1980 accounted for 85 percent of all withdrawals. Surface water also provided domestic supplies for 84 percent of Colorado's population (U.S. Geological Survey, 1986). Because of these large dependencies on surface water, shortages during droughts can affect nearly all citizens and most industries. Droughts also can adversely affect the quality of surface-water supplies; concentrations of detrimental constituents increase during droughts because of lack of flow to dilute the contaminants.
The major floods and droughts described here are those that were of substantial areal extent. In addition, the floods had peak discharges with recurrence intervals of more than 25 years, and the droughts had recurrence intervals of more than 10 years. These major events and other floods of smaller areal extent are listed chronologically in Table 1.
The evaluation of floods and droughts in Colorado, as determined from streamflow records, is limited to the period after 1910 when a few continuously recording streamflow-gaging stations were established. Records for 44 long-term gaging stations on streams having minimal upstream regulation were analyzed to determine the periods and areal extent of floods, and the records for 50 gaging stations were analyzed for droughts. From these groups of stations, six gaging stations were selected to depict floods and droughts. The gaging stations were selected to include a diversity of drainage areas and hydrologic settings and to represent runoff conditions from various parts of Colorado; however, the Great Plains are not well represented because of the few gaging stations in that part of the State. Drainage-basin size upstream from the six gaging stations ranges from 164 to 8,050 square miles, and periods of record range from 55 to 100 years. Streamflow data are collected, stored, and reported by water year (a water year is the 12-month period from October 1 through September 30 and is identified by the calendar year in which it ends).
Table 1. Chronology of major and other memorable floods and droughts in Colorado, 1911- 1988
|Flood or Drought||Date||Area Affected||Recurrence Interval (in years)||Remarks|
|Flood||Oct. 4- 6, 1911||Rio Grande and San Juan River basins||>100||Widespread intense rainfall for 3- 5 days. Widespread damage in southwest.|
|Floods||June 2- 17 and Aug. 2, 1921||North Platte, Yampa, White, Roaring Fork, East, Uncompahgre, and Arkansas River basins||25 to > 100||General statewide rainfall and isolated severe thunderstorms and areas of excessive snowmelt.|
|Drought||1930- 42||Statewide||20 to >25||Regional|
|Floods||July 7 and Sept. 9- 10, 1933||South Platte River basin, Plum Clear, and Bear Creek basins.||50 to 100||Intense localized rainfall.|
|Flood||May 30- June 1, 1935||Kiowa, Bijou, Fountain, and Monument Creek basins, and South Fork Republican River basin.||20 to 70||Locally intense thunderstorms. Deaths, 13; damage in Colorado Springs and Pueblo|
|Flood||Sept. 2- 4, 1938||Bear and Clear Creek basins.||20 to 60||Locally intense thunderstorms.|
|Flood||Apr. 23- 24, 1942||Purgatorie River basin||20 to 40||Intense rainfall combined with snowmelt runoff. Highway and railroad bridges destroyed.|
|Drought||1949- 57||Statewide||10 to >25||Regional|
|Flood||June 4- 16, 1952||Colorado, Yampa, White, and Dolores River basins.||20 to 50||Snowmelt probably combined with rainfall runoff|
|Floods||June 4- July 1, 1957||Arkansas, Roaring Fork, Gunnison, and North Platte River basins.||25 to >100||Snowmelt combined with rainfall runoff.|
|Drought||1958- 70||Statewide||less than 10 to >25||Regional|
|Flood||June 14- 22, 1965||South Platte and Arkansas River basins.||50 to >100||Widespread intense rainfall for several days. Declared major disaster area. Deaths, 24; damage $570 million.|
|Flood||Sept. 5- 17, 1970||San Juan and Dolores River basins||5 to >100||Intense sustained rainfall. Declared a major disaster area. Damage, $2.9 million.|
|Flood||July 31- Aug. 1, 1976||Big Thompson and Cache la Poudre River basins.||5 to >100||Intense localized rainfall for about 3 hours. Declared major disaster area. Deaths, 144; damage, $39 million.|
|Drought||1976- 82||Statewide||less than 10 to >25||Regional|
|Flood||July 15, 1982||Roaring and Fall Rivers||>100||Dam failure. Declared major disaster area. Deaths, 3; damage, $31 million.|
|Floods||June and July, 1983||Colorado, Dolores and White River basins (June), and Bear Creek basin (July).||10 to >100||Snowmelt combined with rainfall runoff|
|Floods||May and June, 1984||Colorado, Gunnison, White, Roaring Fork, Uncompahgre, and Yampa River basins.||10 to >100||Snowmelt combined with rainfall runoff. Declared major disaster area.|
Figure 1. Areal Extent of Floods in Colorado. Click on image to view full size version.
The areal extent and severity of flooding determined from streamflow records from the statewide network are shown in Figure 1. The magnitudes of discharge having 10-year and 100-year recurrence intervals, which were determined by an analysis of the annual flood series at the sites, are also shown. The five floods depicted in Figure 1 were selected on the basis of magnitude and areal extent. and are the most memorable since the early 1900's. Several floods occurred in the late 1800's that were widespread and had large magnitudes (Follansbee and Sawyer, 1948, p. 23-26, 73-79, 114-116, and 123-140), but available data are insufficient to quantify their severities or map their areal extents.
The floods of 1921 are used as a reference when comparing historical flood magnitudes in Colorado. Flooding was severe in the North Platte. Yampa, White, Roaring Fork, East, and Uncompahgre River basins during June 13-17, 1921, as a result of snowmelt and rainfall runoff from low altitudes. Flooding was severe in the Arkansas River basin from Pueblo downstream, including the Fountain Creek drainage, during June 13-17, 1921, as a result of widespread rainfall during June 2-7, 1921. On August 2, 1921, an isolated, intense thunderstorm in the Arkansas River basin upstream from Canon City resulted in the peak discharge of record on the Arkansas River at Canon City. All these floods in 1921 were considered to be part of a single flood for this analysis because of the small areal extent of the individual floods.
Severe flooding caused by snowmelt and rainfall occurred in the upper Arkansas and Roaring Fork River basins from June 29 to July 1, 1957. Floods of less severity affected tributaries of the Gunnison River during June 4-7, and the North Platte River on June 15, 1957.
The most severe Colorado floods of the 20th century were those that affected the South Platte and Arkansas River basins during June 1965. The floods were caused by intense rainfall (as much as 14 inches in a few hours) during June 14-17, following a relatively wet spring (Matthai, 1969, p. B-1).
Flooding in the South Platte River basin began on June 14, near Denver. The flood crest did not pass the most-downstream gaging station on the South Platte River in Colorado until June 20,1965. Matthai (1969, p. B-1) reported that eight deaths were attributed to the June 1965 flood on the South Platte River and total damage was $508 million. of which about 75 percent was in the Denver metropolitan area. Peak discharges at several gaging stations had recurrence intervals that exceeded 100 years. The peak discharge on the South Platte River at Denver was 40,300 ft3/s (cubic feet per second), which is 1.8 times greater than the next largest discharge of record since 1889. Matthai (1969, p. B-36) reported a peak discharge on East Plum Creek near Castle Rock of 126,000 ft3/s from a contributing drainage area of 108 mi2.
During June 17-19, 1965, moderate to severe flooding occurred in the Arkansas River basin (Fig. 1). The flooding was caused by extreme rainfall on June 16 and 17 following 2 days of moderate rainfall; snowmelt was a minor contributor. Widespread rainfall in May and early June created moist antecedent conditions in most areas affected by the flood. The peak discharge of the Purgatoire River at Ninemile Dam, near Higbee on June 18 became the peak discharge of record for that station and has remained the peak discharge of record to the present (1988). Peak discharges during the June 1965 floods in the Arkansas River basin in Colorado, Kansas, and New Mexico were greater than those previously recorded at 48 of 136 other gaging stations as well, and many peak discharges had recurrence intervals that exceeded 100 years. Sixteen lives were lost due to the flooding, and property damage was about $60 million (Snipes and others, 1974, p. D-1).
The flood of July 31-August 1, 1976, on the Big Thompson and Cache la Poudre Rivers, resulted in at least 144 deaths and total damage of about $39 million (McCain and others, 1979, p. 70, 71). Some of the peak discharges on the Big Thompson River were extremely rare; the largest was about four times that having a 100-year recurrence interval. Other peak discharges, especially on the Cache la Poudre River, were not as significant. This flood was produced by 6- 12 inches of rainfall from a storm centered over the downstream part of the Big Thompson River basin during the evening of July 31, 1976. Many of the lives lost were campers who had set their camps near the river.
The flood of July 15, 1982, on the Roaring and Fall Rivers resulted from failure of a 26-foot-high earthen dam that formed a small irrigation storage reservoir at an altitude of about 11,000 feet. According to Jarrett and Costa (1986, p. 6), the most likely cause of dam failure was erosion around the outlet pipe, which eventually weakened the dam. causing a breach. The dam failure released 674 acre-feet of water into the Roaring River, which then reached an estimated peak discharge of 18,000 ft3/s within about 10 minutes. The floodwaters moved downstream into the Fall River and tipped over a concrete gravity dam at a smaller reservoir. Downstream from that reservoir, the peak discharge was estimated to be 16,000 ft3/s or larger. The floodwaters then passed through the resort town of Estes Park and into Lake Estes (altitude about 7,500 feet), where the total volume was contained. The floodwaters moved 12.5 miles from the first reservoir into Lake Estes in about 3 hours and 40 minutes. During that time, the flood caused three deaths and $31 million in damage (Jarrett and Costa. 1986, p. 2).
The floods of June and July 1983 were the direct result of snowmelt combined with minor rainfall runoff at lower altitudes in the drainage basins of the upper Colorado, White, Roaring Fork, Dolores, and San Miguel Rivers and Bear Creek. Some flooding occurred along the Colorado River downstream from the mouth of the Roaring Fork River. The peak discharge of the White River near Meeker had a recurrence interval that exceeded 100 years. Peak discharge on Bear Creek at Morrison occurred on July 22.
The May and June 1984 floods also were the direct result of snowmelt combined with minor rainfall runoff. These floods had a larger areal extent than the floods of June and July 1983. Flooding was severe in the Yampa, White, Colorado, Roaring Fork, Gunnison, and Uncompahgre River basins. No official estimate of damage has been made for this flood, but damage was extensive in areas adjacent to the rivers. Peak discharges for this flood were the maximum of record for the Colorado River near Cameo on May 26 and the White River near Meeker on May 25. The peak discharge at site 4 had a recurrence interval that exceeded 50 years and that at site 5 exceeded 100 years.
Figure 2. Areal Extent of Droughts in Colorado. Click on image to view full size version.
Major droughts were identified by analyzing cumulative departures from long-term average stream discharge at gaging stations operated since the early 1900's. Major droughts occurred during four periods-1930-42, 1949-57, 1958-70, and 1976-82. The areal extent and severity of these droughts are shown in Figure 2. The identification of drought periods is subjective because some gaging station records might show consistently less than normal annual departures at the same time that other records show short-term greater than normal annual departures. This subjective differentiation of droughts also is involved in separating droughts when only about 1 year of intervening greater than normal annual departures can be detected on most records, such as between the 1949-57 drought and the 1958-70 drought. In this instance, the droughts were separated because of the melting of an extremely large snowpack throughout most of the higher mountains in the State. In the spring of 1957, the melting of this snowpack resulted in flood flows in several major river basins. The assumption was made that this period of large runoff ended the 1949-57 drought.
The 1930-42 drought was regional in scope. The recurrence interval was 25 years or more statewide and 10-25 years in some small areas. The length of record of the Colorado River near Cameo is insufficient to define the drought. Other gaging-station records, however, indicate that the drought was severe in the Colorado River basin. The Great Plains in Colorado were more severely affected by the 1930-42 drought than elsewhere in the State, because of the "dust bowl," which was caused by the combination of drought, increased tillage, and strong winds. Agricultural losses were substantial throughout the Great Plains in Colorado.
The 1949-57 drought affected the entire State. The drought had recurrence intervals of between 10 and 25 years in the lower Arkansas River basin; but in other areas, streamflow deficits indicated a drought with a recurrence interval of greater than 25 years. Thomas and others (1963, p. F-l) reported that, in the Colorado River basin in Colorado, the drought initially was not as severe as in the basin downstream from Colorado. However, the severity of the drought increased in the upper basin in 1952, and Colorado was then considered to be in the area of drought.
During 1958-70, a severe drought with a recurrence interval greater than 25 years affected most of the State. Two small, isolated drainage basins were not affected or were affected by droughts having recurrence intervals of less than 10 years. Several other basins or parts of basins were affected by droughts having recurrence intervals that ranged from 10 to 25 years. The duration of the 1958-70 drought ranged from about 6 to 12 years at the selected gaging stations.
The severity and duration of the 1976-82 drought were more variable than those of the previous major droughts. In the description of the hydrologic and human aspects of this regional drought, Matthai (1979, p. 1) considered the drought to last 2 years, 1976-77; however, the analysis of discharge at 50 gaging stations indicated that the minimum duration was slightly longer than 2 years, and the maximum was as long as 13 years.
Colorado Water Conservation Board, 1985, Flood hazard mitigation plan for Colorado: Denver, 234 p.
Follansbee,R., and Sawyer,L.R.,1948, Floods in Colorado: U.S> Geological Survey Water-Supply Paper 997, 151 p.
Jarrett,R.D., and Costa,J.E., 1986, Hydrology, geomorphology, and dam break modeling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures, Larimer County, Colorado: U.S. Geological Survey Professional Paper 1369, 78 p.
Matthai,H.F., 1969, Floods of June 1965 in South Platte River Basin, Colorado, U.S. Geological Survey Water-Supply Paper 1130, 84 p.
McCain,J.F., Hoxit,L.R., Maddox,R.A., and others, 1979, Storm and flood of July 31-August 1, 1976, in the Big Thompson River and Cache la Poudre River basins, Larimer and Weld Counties, Colorado: U.S. Geological Survey Professional Paper 1115, 152 p.
Thomas,H.E. and others, 1963, Effects of drought in the Colorado River basin: U.S. Geological Survey Professional Paper 372-F, 51 p.