Instream flows were developed for two New Hampshire rivers. This talk focuses on the frequency of times when the instream flow and duration are not met (failure), the severity of the water deficit, the characteristics of the failure probability distributions, regionalization of the results, and potential management approaches to attain instream flows. The two rivers in this study are the Lamprey and Souhegan Rivers. Instream flows were developed for various bioperiods through the year. The instream flows are both flow and duration specific for each bioperiod. Natural long term hydrology was developed from today’s gaged flows by removing human disturbances from the records. By comparing the instream flows to the long period flow records, the probability distributions of flows were fitted to the failure characteristics (frequency of occurrence, duration, deficit volume). These probability distributions can then be used to predict extreme events much as is done for flood predictions. In addition, the characteristics of these distributions can be put into a fashion that allows them to be applied regionally. That is, with estimates of watershed size and average streamflow, the effects (failure characteristics) of applying similar instream flows to other streams can be estimated.