The significance of energy transfer between acoustic normal modes as they propagate downslope in a refractive sound channel is investigated. Specifically, two criteria for effective conversion of sound energy from high to low modes are established. In addition, the results from three simulation case studies are presented. These results show that for a low-frequency (70-Hz) bottom-lying sound source, the conversion of energy to the lowest modes is significant when the source depth is shallow. However such conversion does not occur when the depth of a bottom-lying source is near the axis of the deep sound channel.