Flood frequency curves provide the annual probability of exceeding a specific flood flow. Unregulated peak flow frequencies are typically estimated based on a statistical analysis assuming floods are random homogenous events. Bulletin 17B procedures recommend a log-Pearson Type III distribution to fit a curve through observed flood data. However, such statistical analysis is inappropriate for regulated flood flows because they are affected by the flood operation of reservoirs as well as the volumes and peaks of flood hydrographs. A regulated flood frequency curve can be derived from a long unregulated period of flow record based on routing studies and developing a relationship between regulated peak outflow and unregulated inflow peak or volume, often called a peak flow transform. To resolve the interaction of peak flow rate and flood volume, a critical peak duration is often chosen, averaging flow over several days. The unregulated peak flow frequency curve, averaged over a critical duration, is then transformed to produce the regulated peak flow frequency curve. This paper examines the theoretical behavior of regulated peak transforms and provides a short case study within the Feather-Yuba river system. Two operating rules are used to simulate inflow, outflow, and storage within the reservoir and illustrate the development of a peak flow transform. These rules are: 1) optimal peak reduction with perfect foreknowledge of the flood hydrograph; and 2) minimized exceedences of downstream channel capacity. The flow transforms developed using these two operating rules seem likely to bound the range of actual peak flow transforms.