Low-Flow Criteria for Stream Standards

Virginia's 7-day, 10-yr low flow criterion for determining a stream's capacity for waste assimilation is evaluated. The study consisted of five general phases: selection of the gaging stations and the period of record to be investigated, obtaining low-flow summaries and daily discharge data, preparation of frequency curves, preparation of duration curves by a digital computer from the daily discharge data, and evaluation of the 7-day, 10-yr standard based on the percent of time that the resulting design flow was equaled or exceeded during the period studied. Thirty years of continuous daily discharge data from 30 gaging stations located in six drainage basins were studied, and it was found that the 7-day, 10-yr standard is uniform when applied to the areas of Virginia analyzed in this study. The standard provided a design flow that was is every case, except one, equal to or greater than 99% of the daily flows. No advantages were seen to be gained in changing the present 7-day, 10-yr minimum stream-flow standard.