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<br />caused three deaths and $31 million worth of damages in the Estes Park/Rocky <br />Mountain Park area, <br /> <br />STREAf4 GAUGES <br /> <br />In estimating the 100-year frequency floods, (or that for any other long <br />return period) the ideal situation would be to have a stream gauge record of <br />at least equal length in every river basin in the state, From a practical and <br />economic point of view, this situation will never happen even though available <br />information is accumulating. Only a few stream gauging stations in Colorado <br />presently have a long-term period of record suitable for use in a flood flow <br />frequency analysis. <br />Systematic recording of the state's major rivers first began during the <br />early days of water resources development in the first decade after Colorado <br />was admitted to the Union in 1876. The first gauging station in Colorado <br />began recording in June 1881, in the Cache la Poudre River at the mouth of the <br />canyon near Fort Collins. The station operated for only three months that <br />year, but was put back into operation again in May 1883. However, portions <br />of the record taken in 1881 have been found to be unreliable and only monthly <br />discharge was recorded for some of the early periods. <br />Although it would appear that a continuous record is available at this <br />station for over 100 years, the natural flow of the stream has been signifi- <br />cantly affected by transbasin and transmountain diversions above the station <br />for several purposes. This situation is common in Colorado where nearly every <br />drop of the precious water resource is put to some form of beneficial use; but <br />it severely limits the use of this and many other gauges in a flood frequency <br />analysi s, <br />Recent agricultural developments since the early mining days have signi- <br />ficantly altered "natural" drainage patterns throughout the state. Diversions <br />and storage reservoirs obscure flood records and reduce a downstream <br />stations's value in statistically estimating flood discharges for rare events <br />on a regional basis. As the trend in water use steadily changes from agri- <br />cultural to municipal and industrial purposes, the usefulness of some pre- <br />sently long records will have to be evaluated. <br />By the year 1900, or approximately 20 years after the first Colorado <br />gauge was installed, there were only 26 stations operating in the entire state <br />and half of these were on the South Platte River System of the Missouri River <br />Basin. The majority of the stations operating today were installed by the <br />federal government after agreements for their operation and maintenance by <br />state agencies were worked out. <br />A review of Colorado gauging stations in 1973 indicated that only 18 had <br />a record of unregulated peak flows for more than one 50-year period. The <br />longest period of record was only 67 years. A breakdown of peak flow gauging <br />records suitable for use in a regional analysis of each major river basin is <br />summarized in Table 1. The average period of record of all stations with a <br />record of at least ten years was only 24.7 years, <br /> <br />STATISTICAL ANALYSES <br /> <br />Flood events occur randomly in nature; thus, statistics and the laws of <br />probability are practical tools in estimating flood flows, Several different <br />types of probability distributions have been used in the past by hydrologic <br />engineers to fit an annual flood series to a mathematical model. These <br />distributions have included among others, the normal, gamma, Gumbel, and their <br /> <br />5 <br />