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V. The Role of New Storage in Managing Colorado Water: A Basin -by -Basin Assessment A. Introduction Reservoirs have been part of Colorado's water development strategy since the late 1800s, in response to its highly variable streamflows. Streamflow patterns in Colorado are dominated by snowpack accumulation and melting. In a typical year, about 65% of streamflow occurs in May through July, while less than 20% occurs during late summer and less than 5% occurs during winter months, as shown in Figure V.1. Streamflow also varies considerably from year to year as shown in Figure V.2, with drought periods frequently extending over several years. By the 1870s, demands for summer irrigation water began to exceed the reliable streamflow and irrigation companies began building reservoirs to capture excess spring runoff for release later in the irrigation season. By the early 1900s, cities also turned to reservoir development to provide reliable winter season supplies and for drought protection. From the 1880s through the 1920s, most reservoirs built in Colorado, with the exception of Cheesman Reservoir on the South Platte, were modest projects — small off - channel structures filled via feeder ditches — that reflected the engineering and financial limitations of local water users. Construction of Colorado's major on- stream reservoirs in the 1940s through the 1960s was made possible by a combination of the United States' deliberate investment policies in western water development projects, the technical expertise of the Bureau of Reclamation and the growing financial capabilities and political influence of Colorado's larger cities. 2500 2000 1500 3 0 LL E R d 1000 500 Figure V.1: Typical Streamflow Pattern (Cache la Poudre River) 31 0 1 -Oct 1 -Nov 1 -Dec 1 -Jan 1 -Feb 1 -Mar 1 -Apr 1 -May 1 -Jun 1 -Jul 1 -Aug 1 -Sep