Once water quality criteria are derived by states for a designated use of a particular water body and approved by the U.S. Environmental Protection Agency, they become enforceable in-stream water quality standards. On rivers, these standards are usually applied to a designated low stream flow to establish water quality-based effluent limits. The design stream flow used most often is the Q_7–10, the lowest 7-consecutive-day average flow that is expected to occur once in 10 years. By definition, the Q_7–10 is generally exceeded in approximately 99 percent of the historical record. Therefore, setting effluent limits to continuously meet the in-stream standards designed for infrequent low flow periods, regardless of the actual flow conditions, adds a margin of safety to that already provided by the water quality standard.

The objective of this effort is to evaluate the design-stream-flow concept and the general relationships between water quality standards and effluent limits. An approach is presented to estimate the relative biological risks involved with applying instream standards to alternative stream flows. As an example, a predictive assessment of Pennsylvania's cyanide standard for the protection of warmwater fisheries was evaluated, using the lower Monongahela River as a model. Requirements for this assessment included hydrological, biological, and water quality data for the Monongahela River and an evaluation of the toxicological literature on cyanide.

The results of this assessment suggest that in setting standards for cyanide, the Q_7–10 design flow needs to be employed only when the most biologically sensitive portion of the year corresponds with periods of potential low flow. At other times of the year, design flows greater than the Q_7–10 can be used in standard setting. These preliminary findings provide a technically defensible site-specific basis for the dynamic application of water quality standards to more than one design stream flow. Such an approach can be developed by states to protect the most sensitive life-history events of aquatic species occurring during historically low-flow periods while still providing an equivalent degree of protection to the aquatic community during the remaining major periods of the year.