We studied the population dynamics of the smalleye shiner Notropis buccula, an imperiled cyprinid fish that is endemic to the Brazos River, Texas. Abundance, measured as the mean number captured per seine haul, varied by over an order of magnitude-from 0.4 to 4.3-during the study period. Population structure and reproductive biology were studied to obtain the parameter estimates required for construction of an age-structured population dynamics model. We included stream discharge as a driving variable and modeled age-0 survival as a function of that variable. We compared the observed abundance of smalleye shiners with the abundance predicted by the discharge model and with three null parameterizations of the model using the Akaike information criterion. The discharge-related model received the greatest support from catch data and indicated that a discharge of 6.43 m3/s is necessary for maintenance of the smalleye shiner population in the upper Brazos River drainage. Elasticity analysis and sensitivity simulations of the projection matrix indicated that age-0 survival and age-1 fecundity were the most influential parameters in the population dynamics of smalleye shiners. In combination, these two stages accounted for the majority (70%) of the total elasticity of the population growth rate. We used our model to assess the potential effects of a proposed impoundment on the upper Brazos River and conclude that the reservoir could reduce the abundance of smalleye shiners by about 85% and in the long term is likely to cause the extirpation of this species from the impacted area.