Nitrous oxide (N2O) emissions show large variability among dam reservoirs, which makes it difficult to estimate N2O contributions to global greenhouse gases (GHGs). Because river damming alters hydraulic residence time and water depth, the hydraulic load (i.e., the ratio of the mean water depth to the residence time) was hypothesized to control N2O emissions from dam reservoirs. To test this hypothesis, we investigated N2O fluxes and related parameters in the cascade reservoirs along the Wujiang River in Southwest China. The N2O fluxes showed obvious temporal and spatial variations, ranging from -7.86 to 337.22 μmol m-2 d-1, with an average of 12.76 μmol m-2 d-1. Nitrification was the main pathway of N2O production in these reservoirs, and seasonal dissolved oxygen (DO) stratification played an important role in regulating the N2O production. The reservoir N2O flux had a significant negative logarithmic relationship with the hydraulic load, suggesting its control of the N2O emission. This was because the hydraulic load was a prerequisite for regulating the nitrification-denitrification and the DO stratification in the dam reservoirs. This empirical relationship will help to estimate the contribution of reservoir N2O emissions to global GHGs.