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208 National Water Summary 1988-89-Floods and Droughts: STATE SUMMARIES The second precipitation pattern affects the eastern one-half of Colorado during the spring and summer. From the Great Plains to the foothills of the Rocky Mountains, a substantial increase in precipitation begins in early March and continues through June. As temperatures rise, the supply of moisture from the Gulf of Mexico steadily increases over the plains east and southeast of Colorado. Midlatitude frontal systems crossing the region commonly strengthen on the leeward (east) side of the Rocky Mountains and draw this moisture into eastern Colorado. This moisture and increased con- vective activity result in periodic, widespread rainfall and occasion- ally severe thunderstorms east of the mountains. May and June tend to be the wettest months in the northeastern quarter of Colorado. The third precipitation pattern affects the entire State to some extent but is most pronounced in the southern one-half during sum- mer. Subtropical moisture from the Pacific and Atlantic Oceans drifts northward into the southwestern United States each summer. 'Fhis moisture flow, driven by a weak monsoonlikecirculation (Schmidli, 1984, p. 4-5), generally begins in early July, peaks near the beginning of August, and then gradually weakens and moves southward in late summer. The monsoon moisture results in the frequent summer thunderstorms in the southern Rocky Mountains. Tropical cyclones from the Pacific Ocean are a related, but less dependable, moisture source for southern Colorado. From late August through October, moisture from dissipating tropical cyclones occasionally spreads across the Southwest and into the southern part of the State. Although not a reliable contributor to precipitation in Colorado, these storms occasionally produce large quantities of rainfall over the southern Rocky Mountains (Walts, 1972, p. 158-160). At several locations in southwestern Colorado, these storms arrive frequently enough to make October the wettest month. In addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, from which moisture evaporate~ into the atmosphere. Typically, as a moisture-laden ocean airmass moves inland. it is modified to include some water that has been recycled one or more times through the land-vegetation-air interface. Floods occur primarily from April through October. Cl"ima- tologically, there are three principal causes for floods in Colorado: Intense local thunderstorms. Intense thunderstorms that commonly occur from May until early September can cause flash floods anywhere in the State; however, storms in higher mountains generally have smaller rainfall intensities and limited areal extent, and floods in mountainous areas are most likely to occur at altitudes below 7.500 feet (Jarrett and Costa. 1988), Intense widespread rainfall. Widespread rains are most likely from April to mid-June and are limited to the Great Plains and eastern foothills. However, similar rains occur in the southwestern mountains, es- pecially in September and October. Lo- calized, intense thunderstorms often ac- company these large-scale storms. Snowmelt. From late April through late June, streamflows are greater than normal in the many rivers that originate in the mountains. Major flooding is most likely when excessive late-winter snowfalls and low temperatures maintain a deep snow- pack throughout a large range of altitudes late into the spring (Shafer and others, 1984). Spring rain on snowpack commonly is considered to be a major flood threat, but this condition seldom occurs in Colorado. Figure 2. Mechanisms that cause drought are morc complex than those that cause tloods. Droughts occur when any or all of the major moisture-delivery patterns are disrupted by large-scale atmospheric changes. When moisture movement is lessened and the frequency of frontal systems crossing the central Rocky Mountains is decreased. such as when large high-pressure ridges develop over the western Cnited States, drought is most likely to develop. MAJOR FLOODS AND DROUGHTS From 20 to 30 floods of some significance occur somewhere in Colorado every year. Annual flood losses in Colorado have av- eraged 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 tlooding in Colo- rado. Since Colorado became a State in 1876, floods have killed at least 350 people and caused cumulative flood losses of about 51.7 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 popu- lation (U.S. Geological Survey, 1986). Because of these large de- pendencies 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 detri- mental constituents increase duri ng 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 chronologi- cally in table 1; rivers and cities are shown in figure 2. The evaluation of floods and droughts in Colorado, as deter- mined from streamflow records, is limited to the period after 1910 when a few continuously recording streamflow-gaging stations were ~ 100 MILES , ;" 100 KILOMETERS Selected geographic features, Colorado. ~ -,