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For Coal Creek SLA used the Environmental Protection Agency SWMM (Reference 12) in determining discharges for the original report. SLA assumed that all irrigation ditches and canals would already be full from upstream runoff and would not have the capacity to intercept additional runoff. The Natural Resources Conservation Service (NRCS) found that one exception to this assumption is Windsor Ditch, which crosses the basin upstream of the Town of Wellington. The NRCS determined that 200 cfs would be diverted from Coal Creek by Windsor Ditch. Discharges for Dry Creek were computed using the SWMM (Reference 12). The drainage basin was divided into 190 subbasins that drained into 139 channel sections. Basins were selected so that hydrographs would be available at critical design points, particularly at irrigation canal crossings and at onstream reservoirs (Reference 1). Precipitation information was obtained from the NOAA Atlas (Reference 18). The 0.2-percent annual chance rainfall, not available from the atlas, was extrapolated assuming a Gumbel probability distribution (Reference 1). Discharges for Fish Creek and the Fall River were computed based upon records of stream gages located on the two streams. The Fish Creek gage has 30 years of record, and the Fall River gage has 9 years of record. These records were analyzed using a log-Pearson Type III distribution as recommended in U.S. Water Resources Council Bulletin 17 (Reference 11). These discharges were weighted with those obtained using regression equations from the Colorado Water Conservation Board's Technical Manual No.1 (Reference 10). The hydrologic study for Spring Creek completed by Anderson Consulting Engineers, Inc., was conducted in 2003 (Reference 72). MODSWMM was used to determine peak discharges for the 10- and I-percent annual chance floods. The study was updated to reflect current drainage patterns, revise detention pond routing parameters, and improve modeling techniques, if applicable. Peak discharge-drainage area relationships for streams studied in detail are shown in Table 1. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of flooding from the sources studied were carried out to provide estimates of the elevations of floods of the selected recurrence intervals. Users should be aware that flood elevations shown on the Flood Insurance Rate Map (FIRM) represent rounded whole-foot elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the Floodway Data table in the FIS report. Flood elevations shown on the FIRM are primarily intended for flood insurance rating purposes. For construction and/or floodplain management purposes, users are cautioned to use the flood elevation data presented in this PIS report in conjunction with the data shown on the FIRM. Cross sections for the Big Thompson River and its tributaries were taken from topographic maps at a scale of 1 :2,400, with a contour interval of 2 feet (References 19, 20, and 21) and field surveys. These tributaries include the North Fork Big Thompson River, Big Thompson River Tributaries 1, 2, 3, and 4, Long Gulch, Rabbit Gulch, Quillan Gulch, 21