This paper presents the steps involved and the methodologies employed in the first phase of the South Carolina Surface Water Assessment - development of extended and unimpaired streamflow estimates based on USGS gage data in the Saluda basin. Streamflow data are first adjusted to remove effects of anthropogenic impairments. Adjustments are made for reservoirs, withdrawals, and discharges based on available documentation. Where documentation is insufficient, hindcasting methods are used. The resulting datasets are called unimpaired flows (UIFs).

The UIFs are then extended in time from 1925, the starting date of the first continuous stream discharge data available in the basin, through 2013. Candidate reference gages for each short-record gage are selected based on a qualitative assessment. Area ratio and Maintenance of Variance Extension (Hirsch, 1982) methods are applied. Statistical and graphical evaluation of the extension results is followed by composition of extended UIFs.