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Water-surface profiles for the Colorado River were calculated with the use of the USACE HEC-RAS step-backwater program (Reference 10). Cross-section data for the Colorado River were developed from topographic maps prepared by Analytical Surveys, Inc. (Reference 11). All bridges were field surveyed to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (See Section 4.2), selected cross-section locations are also shown on the FIRM. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only ifhydraulic 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 used for newly created or revised FIS reports and FIRMs was the NGVD. With the completion of the North American Vertical Datum of 1988 (NA VD), many FIS reports and FIRMs are now prepared using NA VD as the referenced vertical datum. Flood elevations shown in this FIS report and on the FIRM are referenced to the NGVD. These flood elevations must be compared to structure and ground elevations referenced to the same vertical datum. For information regarding conversion between the NGVD and NA VD, visit the National Geodetic Survey (NOS) website at www.ngs.noaa.gov, or contact the NGS at the following address: NOS Information Services, NOAA, N/NGS12 National Geodetic Survey SSMC-3, #9202 1315 East-West Highway Silver Spring, MD 20910-3282 Fax: (301) 713-4172, or Telephone: (301) 713-3242 Temporary vertical monuments are often established during the preparation of a flood hazard analysis for the purpose of establishing local vertical control. Although these monuments are not shown on the FIRM, they may be found in the Technical Support Data Notebook associated with the FIS report and FIRM for this community. Interested individuals may contact FEMA to access these data. To obtain current elevation, description, and/or location information for benchmarks shown on this map, please contact the Information Services Branch of the NOS at (301) 713-3242, or visit their website at www.ngs.noaa.gov. 5