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The mechanics of a structural failure depends on the type of dam and the mode of failure, Earthen dams, the most common type of structure in Colorado, typically breach during failure. Flood waters are relea::;ed through the breach at a rate controlled by the size of a widening opening and a decreasing water level in the reservoir. Dam failure floods due to structural deficiencies are characterized by a sudden rise in stream level and relatively short duration similar to a thunderstorm flood. They can occur at any time, but earthen dams appear to be most susceptible to structural failure during the fall and spring freezing and thawing cycles, There are approximately 27,000 dams in the State of Colorado. This includes 2,249 dams which are under the jurisdiction of the State Engineer, several thousand low dams for smallrcapacity reservoirs known as "Livestock Water Tanks" (which are~.,.normallY inspected), and potential artificial impoundments created by highway embankments constructed across drainageways. A dam must be at least 10 feet high or its reservoir must have a surface area of at least 20 acres or a storage capacity of at least 1,000 acre-feet in order to to fall under the review of the state's dam safety program. Of the 2,249 inspected dams, 120 are federally owned, and 2,129 non-federally owned. Although few lives have been lost from dam property damage has been high, There have been as known dam failures in Colorado since 1890 (see The failure of the Lower Latham Reservoir Dam in subsequent flooding in the town of Kersey, Weld County, resulted in a Presidential Major Disaster Declaration. fail ures, least 130 Appendix) . 1973 and Colorado The earliest recorded dam failure flood in the Estes Park region occured on May 25, 1951 when Lilly Lake dam failed sending flood waters down Fish Creek and into Lake Estes. In June 1965, a flood ocurred on Clay Creek in Prowers County from the failure of an earthen dam being contructed by the Colorado Game, Fish, and Parks Commission. This dam failure flood resulted in an important legal controversy known as the Barr Case. This case was finally decided in 1972 by the Colorado Supreme Court which recognized the concept of probable maximum flood as a predictable and foreseeable standard for spillway design purposes, The most unusual dam failure flood in Colorado is probably the complete draining of Lake Emma, a natural lake located high in the San Juan mountains above Silverton, Colorado. On June 4, 1979, water was able to flush thrQugh a network of tunnels in an abandoned mine extending under the lake. -40-