Abstract: This study uses Pettitt test and Mann-Kendall test to analyze the inconsistency in the sequence of annual peak flood into the Ken Swart reservoir, conducts consistency correction by the decomposition-synthesis theory, and calculates the fuzzy risk rates of dam overtopping under the conditions of the past and present conditions through risk analysis using the method of the right-angled trapezoidal fuzzy numbers. Results show that in this sequence, a change point occurred in 1993, jumping is its major form of variation, and its overall increasing trend is insignificant. In the present condition, the upper limit estimates of the risk rates at different significant level α are larger than those in the past condition, indicating that the snowmelt floods, affected by the significant increase in temperature over the basin, raise the risk of reservoir failure. The results would help the efficient utilization of flood water resources increasing of flood control benefit, and security of downstream flood protection area.