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DRAFT Run Gulch, French Creek, and Illinois Gulch. These eleven drainages range in size from 0.73 to 11.1 square miles and contribute a total of 35.0 square miles of drainage area to the Blue River. In addition to these tributaries there are 2.5 square miles of drainage area contributed to the Blue River within the MPA by small, local drainages. Discharges for the 2-, 5-, 10-, 25-, 50-, and 100-year events were determined for the major tributaries utilizing the Army Corps of Engineers HEC-1 model. Lehman Gulch, Sawmill Creek and Illinois GulCh have previously had 10-, 50-, 100- and 500-year discharges determined by FEMA. For regulatory purposes, the FEMA discharges govern and as a minimum, must be adhered to. This study modelled these basins for two purposes, first to provide not only peak discharges, but entire hydrographs and to help calibrate the model parameters for other basins. In other words, the parameters on Illinois Gulch, Sawmill creek and Lehman Gulch were adjusted, within a reasonable range, to produce discharges close to those previously defined by FEMA. The calibrated parameters were then considered in developing the watershed parameters for the other basins. The main parameters adjusted in the calibration were those controlling rainfall loss and flow resistance. Methodolocrv The HEC-1 computer model simulates the basin runoff response to precipitation by modelling the basin as an interconnected system of hydrologic and hydraulic components. Each system component models a process in the rainfall-runoff behavior withn a subbasin. A component may represent a surface runoff unit, a stream channel unit, or a reservoir. Each component is mathematically represented by a set of parameters which approximate the hydrologic characteristics of the unit, and by a series of mathematical relations which describe the physical processes governing the hydrologic response to precipitation. The outcome of the modeling process is the computation of stream flow hydrographs at locations throughout the watershed. The following processes in the hydrograph cycle are modeled in HEC- 1: Rainfall Interception/Infiltration/Runoff OVerland Flow Channel Flow Rainfall was modeled by developing a hyetograph for the 24-hour storm representing the return period of interest. Hyetographs were developed using the NOAA Atlas II, Volume 3, Colorado, 1973. The interception/infilration process was approximated by using SCS (Soil Conservation Service) curve numbers. Although HEC-1 has 10