REGULATED RIVERS: RESEARCH & MANAGEMENT Regul. Riers: Res. Mgmt. 15: 273–288 (1999) ESTIMATION OF GEOMORPHICALLY SIGNIFICANT FLOWS IN ALPINE STREAMS OF THE ROCKY MOUNTAINS, COLORADO (USA) NICOLA SURIAN a AND E.D. ANDREWS b, * a Uniersity of Padoa, Dipartimento di Geologia, Paleontologia e Geofisica, Via Rudena 3, 35123 Padoa, Italy b U.S. Geological Surey, 3215 Marine Street, Boulder, CO 80303, USA ABSTRACT 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 9% for the 10% exceedance flow, to 11% 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 m 3 /s), and basin relief with an uncertainty of 23% for the 10% exceedance flow to 21% 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. KEY WORDS: regional streamflow analysis; flow duration; floods; regression equations; alpine streams; ungauged sites; Rocky Mountains; Colorado INTRODUCTION Hydrological and geomorphological studies, as well as effective management of water resources, fre- quently require streamflow information at locations where gauging station records do not exist. Thus, methods and procedures to estimate streamflow magnitude and duration at ungauged sites are essential. A regional analysis of streamflow attempts to extend existing records in space, and transfer streamflow characteristics recorded at gauging stations to ungauged sites (Riggs et al., 1980). The principal objective of this study was to determine whether the relatively high, infrequent, but geomorphically and ecologically important flows can be accurately estimated by regional flow duration equations in the Rocky Mountains of Central Colorado. Regional regression equations will be derived to estimate streamflow duration and flood frequency in high mountain streams 2300 m in the Rocky Mountains. This study considers a region with relatively high runoff and abundant water resources. The region contributes more than 20% of the entire runoff of the Colorado River, although it represents 2.5% of the drainage area. Over the past century, the water resources of this region have been extensively developed through the construction of many reservoirs and transbasin diversions. Most streams with * Correspondence to: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA. E-mail: eandrews@usgs.gov Contract/grant sponsor: University of Padova Contract/grant sponsor: US Forest Service, Stream Technology Center, Ft. Collins, Colorado This article is a US government work and is in the public domain in the United States. Receied 10 October 1997 Reised 13 July 1998 Accepted 11 August 1998