Increasing water demands have directed new interest toward methods for defining water supply system performance. These demands include not only traditional municipal, industrial, and agricutural demands, but instream flow uses such as power production and fish flow maintenance. This paper suggests procedures for evaluating water supply performance when instream flows for fish are required of a municipal water supply. A computer simulation model is used with synthetic inflow sequences to investigate indices of system performance for the Seattle, Washington water supply system. A review of techniques used to establish instream flow requirements is presented with a history of instream flow requirements established for the Seattle supply. The trade-offs between values of instream flows and municipal water supply performance are presented to quantitatively illustrate the impacts of instream requirements.