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National Water Summary 1988-89-Floods and Droughts: COLORADO 207 . Colorado's mountainous terrain and midlatitude interior- continental location result in a diverse and complex climate. Mois- ture for precipitation comes from the Pacific Ocean, the Atlantic Ocean, and the Gulf of Mexico. Fluctuations in the dominant pre- cipitation patterns have resulted in severe floods and long-duration droughts. Annual property damage from flood losses in Colorado averages about S 14 million. Cumulative flood losses since statehood in 1876 are estimated to be about $1.7 billion. The most devastating floods of this century in terms of total damage were the June 1965 floods in the South Platte and Arkansas River basins. These floods were the result of intense rainfall for several days following a wet spring. Many of the measured peak discharges had recurrence in- tervals that exceeded 100 years. The most severe flood in Colorado since about 1900, in terms of loss of life, was that of July 31- August I, 1976, on the Big Thompson River. This flood was caused by an intense, localized thunderstorm that lasted a few hours and re- sulted in 144 deaths and $39 million in total damage. The four major Colorado droughts of this century, as deter- mined from periods of generally less than average streamflow vol- umes, occurred during 1930-42,1949-57,1958-70, and 1976-82. These droughts were statewide and had recurrence imervals greater than 25 years. Floods and droughts directly affect surface-water quality be- cause of the flushing of contaminants into the streams during floods and the lack of dilution during droughts. Also, the traveltime for contaminants is considerably decreased during periods of high flow. As a result of these effects, sport fisheries generally are adversely affected, which in turn affects tourism, an important industry in Colorado. GENERAL CLIMATOLOGY The Colorado Water Conservation Board, by State statute, is responsible for tlood-plain management, including coordination of the National Flood Insurance Program. The Office of the State Engineer and the National Weather Service operate a statewide flood- warning system consisting of a network of 78 satellite-linked streamflow-gaging stations. This sy~tem automatically alerts key personnel if flooding is probable. The Colorado Drought Response Plan, which was created in 198 I in response to signs of drought, consists of an assessment phase and a response phase. In the assessment phase, various water- availability indicators are reviewed monthly. If drought conditions are indicated, the response phase is activated to evaluate the drought ingreater detail. Colorado's midlatitude interior-continental location and high- altitude mountainous terrain combine to produce a complex and di- verse climate. Annual precipitation ranges from about 7 inches in south-central Colorado to about 60 inches in the mountains east of Steamboat Springs (Doesken and others, 1984, p. 1). Seasonal. large-scale atmospheric circulation interacts with the mountainous topography to produce three major precipitation patterns in the State (fig. I). The most important of these, in terms of water resources, is the midwinter pattern. Throughout the winter. the primary sources of moisture are frontal systems from the Pacific Ocean that are directed by the polar jetstream into Colorado from the northwest, west, and southwest. These airmasses alternately collide with, replace, or are re- placed by drier. continental airmasses. In Colorado, the major ranges of the Rocky Mountains. which are oriented primarily north- south, present a formidable ob- stacle to eastward-moving mois- ture. As the air is forced to rise over the mountains, a substantial oro- graphic component is added to the winter precipitation pattern. In general, winter precipitation in- creases steadily with altitude west of the Continental Divide and de- creases sharply east of the divide. For most of the higher mountains and for parts of Colorado's western valleys, winter is the wettest part of the year, whereas east of the mountains, areas receive very little precipitation in winter. Most of the winter precipitation in the moun- tains remains as accumulated snowpack until melting begins in the spring. The winter pattern con- tributes most to statewide runoff and surface-water supplies, even though substantial snowfall is limited to high-altitude areas that constitute less than 15 percent of the State's land area. ~. 1 .' Figure 1. Principal sources and patterns of delivery of moisture into Colorado. Size of arrow implies relative contribution of moisture from source shown. (Source: Data from Douglas R. Clark and Andrea Lage, Wisconsin Geological and Natural History Survey.]