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The Pinon Canyon Dam, completed in 1954 by the U. S. Army Corps of Engineers, protects a highly developed area of about 40 acres in the northwestern and central sections of the city from floods originating in the precipitous pinon Canyon. This earthfill detention dam provides regulation of storms with a recurrence interval greater than 100 years. The ungated outlet drains the reservoir within a reasonable time and the ungated out- let discharge does not appreciably affect floods from the uncontrolled area below the dam. The Soil Conservation Service constructed dams on Carbon and Fishers Peak Arroyos to prevent recurrent damages to the watershed and the City of Trinidad. The structures are of earth construction, with ungated spillways that . provide regulation of storms with a recurrence interval greater than 100 years. An earthfill dike and unlined vegetative channel approximately 2000 feet in length divert a small tributary of Fishers Peak Arroyo into the Fishers Peak Arroyo flood retarding structure. 3.0 ENGINEERING METHODS For streams studied in detail in Trinidad, standard hydrolog- ic and hydraulic study methods were used to determine the flood hazard data required for this study. Floods having recurrence intervals of 10, 50, 100, and 500 years have been selected as having special significance for flood- plain management and for flood insurance premium rates. The analyses reported here reflect current conditions in the watersheds of the streams, but assume full operation of the Trinidad Lake Project. 3.1 Hydrologic Analyses Hydrologic analyses were carried out' to establish the peak discharge-frequency relationships for floods of the selected recurrence intervals for each stream studied in detail. Flood frequency data for the study area were developed from discharge-frequency relation- ships of historic floods and from stream gage records. The twelve years of stream gage records available for Frijole Creek, a nearby stream, were analyzed using the procedures outlined in the Water Resources Council Bulletin No. 17 (Reference 2) and in Statistical ~ethods in Hydrology (Reference 3) to determine the peak fre- quency curves for Frijole Creek. The peak frequency curves for Frijole Creek were then adjusted by a drain- age area ratio (Reference 4) in order to derive the frequency curves for the streams studied in detail in this report. 7 L. ..d