Analysis of flood risks generated by failure or poor operation in reservoirs has been studied over the years. Conventionally, to assess risk of flooding downstream of a dam, spillways operation is analyzed associated a return period of 10 000 years, considering as initial or conservation level in the reservoir, the normal operating water level (NWL). However, in practice, conservation level differs below the NWL. In present article, La Boca dam reservoir is studied located upstream El Cuchillo Dam in the state of Nuevo León (Mexico), analyzing and updating the design floods associated to different return periods. To define the frequent levels in the reservoir, the analytical simulation of the reservoir is optimized. These levels will be the initial conditions in reservoir flooding analysis, obtaining the probabilities of exceedance associated to maximum flows of discharge. The probable risk from floods downstream of a dam is estimated as the sum of the product of the probability of exceedance respect to probability of occurrence. The proposed methodology allows estimate accurate flood risk results without overestimating costs, generating considerable savings in possible areas of affectation.