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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak <br />discharge-frequency relationships for floods of the selected recurrence <br />intervals for each stream studied in detail in the county. <br /> <br />Discharge-frequency analyses for Animas River, Hermosa Creek, Lighter Creek <br />(downstream of Wildcat Canyon), Junction Creek, Dry Gulch Creek, Vallecito <br />Creek, and Grimes Creek were prepared by the U.S. Army Corps of Engineers <br />(Reference 1, 2, and 3) and reviewed by the study contractor. <br /> <br />Methods employed in the U.S. Army Corps of Engineers discharge-frequency <br />analyses included streamflow record analysis and the Synthetic Hydrograph <br />method. Discharge-frequency analyses for the Animas River and Lightner Creek <br />(downstream of Wildcat Canyon) were based on streamflow records of rain and <br />snowmelt floods and on data based on steamflow records of rain and snowmelt <br />floods and on data from historic storms. Discharge-frequency analyses for <br />Junction Creek were based on streamflow records and on data developed from <br />similar stream basins in the region. Discharge-frequency analyses for Dry <br />Gulch Creek were based on the Synthetic Hydrograph method because historic <br />floodflow and precipitation data were not available. For Grimes and <br />Va:lecito Creeks, discharge-frequency analyses were based on records from the <br />C.S. Geological Survey gagi~g station near Bayfield on Vallecito Creek and <br />several long-terw stream gages in the vicinity having similar basin <br />c~aracteristics. <br /> <br />Discha~ge-frequency analyses for Lighter Creek (upstream of Wildcat Canyon) <br />and Los Pinos River were prepared oy the study contractor. <br />Discharge-frequency analyses for Lightner Creek (upstream of Wildcat Canyon) <br />were based on ~egio~al peak disc~arge-freque~cy relationships. Ten gaging <br />stations in the Animas River basin were selected to establish the regional <br />curves. For each selected gaging station, rain and snowmelt events are <br />fitted separately with log-Pearson Type III distribution (Reference 4) and <br />then combined statistically into one frequency curve. Regional relationships <br />of basin area to peak discharge for the 10-, 50-, 100-, and 50Q-year floods <br />were developed through regression analysis. <br /> <br />Discharge-frequency analyses for Los Pinos River below Vallecito Dam at <br />Bayfield were based on the operational records of Vallecito Reservoir, from <br />1943 to 1973 (Reference 5), and regional relationships developed for the <br />Upper San Juan River basin. The operational records of Vallecito Reservoir <br />were used to establish the peak discharge-frequency relationships at the <br />outlet of the reservoir. Peak discharges of selected recurrence intervals at <br />Bayfield were determined by adding discharges of selected recurrence <br />intervals produced by the drainage area downstream of the Vallecito Reservoir <br />to the corresponding peak discharges at the reservoir outlet. Discharges of <br />selected recurrence intervals contributed from drainage areas downstream of <br />Vallecito Reservoir were determined by applying the regional relationships <br />established for the Opper San Juan River basin. <br /> <br />Discharge-frequency values for the Animas River and Hermosa Creek in the <br />Vicinity of He~mosa were taken directly from the U.S. Army Corps of <br />Engineers, Sacramento District, and based on streamflow records of rain and <br />snowmelt floods (Reference 6}. <br /> <br />Peak discharge-drainage area relationships from Animas River, Hermosa Creek, <br />Junction Creek, Dry Gulch Creek, Lightner Creek, Los Pinos River, Vallecito <br />Creek, and Grimes Creek are shown in Table 1. <br /> <br />8 <br />