Estimation of geomorphically significant flows in alpine streams of the Rocky Mountains, Colorado (U.S.A.)

Streamflows recorded at 24 gauging stations in the Rocky Mountains of Colorado were analyzed to derive regional regression equations for estimating the natural flow duration and flood frequency in reaches where the natural flows are unknown or have been altered by diversion or regulation. The principal objective of this analysis is to determine whether the relatively high, infrequent, but geomorphically and ecologically important flows in the Rocky Mountains can be accurately estimated by regional flow duration equations. The region considered in this study is an area of relatively abundant runoff, and, consequently, intense water resources development. The specific streams analyzed here, however, are unaltered and remain nearly pristine. Regional flow duration equations are derived for two situations. When the mean annual discharge is known, flows ]10% of the time can be estimated with an uncertainty of 99% for the 10% exceedance flow, to 911% for the 1.0% exceedance flow. When the mean annual discharge is unknown, the relatively high, infrequent flow can be estimated using the mean basin precipitation rate (in m3:s), and basin relief with an uncertainty of 923% for the 10% exceedance flow to 921% for the 1.0% exeedance flow. The uncertainty in estimated discharges using the equations derived in this analysis is substantially smaller than has been previously reported, especially for the geomorphically significant flows which are relatively large and infrequent. The improvement is due primarily to the quality of streamflow records analyzed and a well-defined hydrologic region.