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Floodflow frequency data for the Rio Grande was based on statistical analyses of stage-discharge records covering a 73-year period of gaging stations operated by the U. S. Geological Survey (Reference 2). These analyses followed the standard log-Pearson Type III method (Reference 3). Discharge for the 50Q-year flo~d was determined by straight-line extrapolation of a single-log graph of flood discharges computed for frequencies up to 100 years. Flow frequency parameters for Rock Creek were developed from pro- cedures outlined in a u.s. Geological Survey manual (Reference 4). A regional frequency study was used to develop basic discharge- frequency relationships for North and South Zapata Creeks. Sixteen stream gaging stations with similar characteristics were selected for frequency analyses. Discharge-frequency relationships for each station were computed by methods outlined by the Water Resources Council (Reference 3). Frequency-Discharge, Drainage Area Curves for the Rio Grande, Rock Creek, North Zapata Creek, and South Zapata Creek are shown in Figure 4. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the unincor- porated areas of Alarnosa County were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along the stream segment studied in detail. Cross section data for the Rio Grande was obtained from aerial sur- veys and aerial photography (Reference 5). All bridges and culverts in the study areas were 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 is computed (Section 4.2) , selected cross section locations are also shown on the Floodway Map (Exhibit 2). Channel and overbank roughness coefficients (Manning's IIn") for the Rio Grande were assigned on the basis of field inspections. Rough- ness values vary from 0.035 to,O.04Q for the channel, and 0.08 for the overbanks. Water-surface elevations for the Rio Grande were developed using the U.S~ Army Corps of Engineers HEC-2 step-backwater computer model (Reference 6). Flood profiles were drawn showing computed water- surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). Starting elevations for the Rio Grande were determined by the slope-area method. 8