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EXECUTIVE SUMMARY The publication entitled Guidelines for Determining lOO-Year Flood Flows for Approximate Floodplains in Colorado (Guidelines) was prepared by Colorado Water Conservation Board (CWCB) staff as a tool for estimating IOO-year flood discharges for approximate floodplains where detailed engineering analyses are limited or unavailable. The Guidelines are designed to provide a streamlined hydrologic procedure for use in the review and designation of approximate floodplain studies and mapping in Colorado, The Guidelines facilitate the estimation of IOO-year flood discharges for approximate floodplains as required by the CWCB's technical standards. Many of the Flood Insurance Rate Maps (FIRM's) and Flood Hazard Boundary Maps (FHBM's) prepared by the Federal Emergency Management Agency (FEMA) and Flood Insurance Administration (FIA) include approximate floodplain delineations. Those approximate delineations do not have detailed hydrologic information to accompany them and were therefore not previously designated by the CWCB. However, Colorado statutes require that floodplain information to be used by local governments for land use and regulatory pmposes must first be designated and approved by the CWCB. The Guidelines will permit fulfillment of that statutory requirement by allowing approximate floodplain delineations to meet the CWCB's hydrologic technical requirements for designation and approval. The methodology in the Guidelines utilizes IOO-year flow values and drainage areas from existing detailed floodplain studies. Due to the great hydrologic diversity of Colorado's streams, the state was divided into major river basins, and then further divided into subregions of hydrologic homogeneity. Regression equations were developed in order to relate I DO-year peak flow values to the drainage areas for each hydrologic subregion within Colorado, By using these regression techniques, the Guidelines methodology will allow hydrology for approximate floodplain delineations to be related to detailed published hydrology from floodplain studies which have already met the CWCB's technical requirements for designation and approval. At this time, the Guidelines for eastern Colorado are completed, while work in western Colorado is in progress. The use of the Guidelines is restricted to the estimation of I DO-year flood discharge values for streams where approximate floodplain information or no information is available., The Guidelines are not to be used for the design of hydraulic structures or for the preparation of detailed floodplain studies. Detailed hydrologic and hydraulic analyses should be performed for any planned development involving residential subdivisions, commercial structures, industrial structures, public facilities (roads, bridges, treatment plants, etc.), and flood control facilities. If a detailed study is subsequently completed on a stream reach which has been designated as approximate, the new detailed floodplain information will supersede the previously designated approximate information.