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<br />3,1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for floods of the selected recurrence inter- <br />vals for each stream studied in detail in the county. <br /> <br />Discharge-frequency analyses for Animas River, Hermosa Creek, <br />Lightner Creek (downstream of Wildcat Canyon), Junction Creek, Dry <br />Gulch Creek, Vallecito Creek, and Grimes Creek were prepared by the <br />U.S. Army Corps of Engineers (References 1, 2, and 3) and reviewed <br />by the study contractor, <br /> <br />, <br /> <br />Methods employed in the U.S. Army Corps of Engineers discharge- <br />frequency analyses included streamflow record analysis and the <br />Synthetic Hydrograph method. Discharge-frequency analyses for the <br />Animas River and Lightner Creek (downstream of Wildcat Canyon) were <br />based on streamflow records of rain and snowmelt floods and on data <br />from historic storms. Discharge-frequency analyses for Junction <br />Creek were based on streamflow records and on data developed from <br />similar stream basins in the region. Discharge-frequency analyses <br />for Dry Gulch Creek were based on the Synthetic Hydrograph method <br />because historic floodflow and precip,itation data were not available. <br />For Grimes and Vallecito Creeks, discharge-frequency analyses were <br />based on records from the U.S. Geological Survey gaging station near <br />Bayfield on Vallecito Creek and several long-term stream gages in <br />the vicinity having similar basin characteristics. <br /> <br />- <br /> <br />Discharge-frequency analyses for Lightner Creek (upstream of Wildcat <br />Canyon) and Los Pinos River were prepared by the study contractor, <br />Discharge-frequency analyses for Lightner Creek (upstream of Wildcat <br />Canyon) were based on regional peak discharge-frequency relationships, <br />Ten gaging stations in the Animas River basin were selected to <br />establish the regional curves. For each selected gaging station, <br />rain and snowmelt events are fitted separately with log-Pearson Type <br />III distribution (Reference 4) and th~n combined statistically into <br />one frequency curve. Regional relationships of basin area to peak <br />discharge for the 10-, 50-, 100-, and 500-year floods were developed <br />through regression analysis. <br /> <br />Discharge-frequency analyses for Los Pinos River below Vallecito Dam <br />at Bayfield were based on the operational records. of Vallecito <br />Reservoir, from 1943 to 1973 (Reference 5), and regional relation- <br />ships developed for the Upper San Juan River basin. The operational <br />records of Vallecito Reservoir were used to establish the peak <br />discharge-frequency relationships at the outlet of the reservoir. <br />Peak discharges of selected recurrence intervals at Bayfield were <br />determined by adding discharges of selected recurrence intervals <br />produced by the drainage area downstream of the Vallecito Reservoir <br />to the corresponding peak discharges at the reservoir outlet. <br />Discharges of selected recurrence intervals contributed from drainage <br />areas downstream of Vallecito Reservoir were determined by applying the <br />regional relationships established for the Upper San Juan River basin. <br /> <br />8 <br />