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The starting WSEL for Fryingpan River was obtained from a flood study along the Frymgpan River through the Town of Basalt (Reference 26). Along Taylor Creek, the starting WSELs were taken from those elevations calculated for the Fryingpan River. For the original Brush Creek Study, starting WSELs were calculated using critical depth at the mouth of Brush Creek, however, the starting water surface elevation for the August 2003 study of Brush Creek was computed based on a known WSEL. Starting WSELs for Turkey Creek were derived from the calculated flood elevations along the Eagle River near the Town of Red Cliff. Starting WSELs for Gypsum Creek and Gore Creek were calculated using the slope-area method. Starting WSELs for the Roaring Fork River were calculated using critical depth at the beginning and end of the study. This is a reasonable assumption because of the steep gradient. The USACE Hydraulic Engineering Center-River Analysis System (HEC-RAS) computer. program was used to determine WSELs for the August 2004 Eagle River study, the August 2003. Brush Creek study, Gypsum Creek, Gore Creek and its tributaries, and the Roaring Fork River. WSELs of the other flooding sources in Eagle County were computed through use of the USACE HEC-2 step-backwater computer program (Reference 26). A channel roughness value of 0.035 was selected for Gypsum Creek; floodplain roughness values ranged from 0.040 to 0.090. Roughness coefficients for Brush Creek range from 0.03 to 0.035 for the channel and 0.030 to 0.060 for the overbank areas. Manning's "n" values for the Colorado River range from 0.030 to 0.070 for the main channel and 0.050 to 0.080 for the overbank areas. Channel and overbank roughness for the Eagle River were determined from field investigations and use of the ortho-rectified aerial imagery. The values for the Eagle River channel vary from 0.035 to 0.050 and 0.040 to 0.100 for the overbank areas. The Manning's "n" values for Gore Creek and its tributaries were determined by field inspection by the SC and were estimated to be 0.04 for the main channel and 0.05 to 0.08 for the overbank areas. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 3.3 Vertical Datum All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum in use for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD). With the finalization of the North American Vertical Datum of 1988 (NA VD), many FIS reports and FIRMs are being prepared using NA VD as the referenced vertical datum. 26