Comminution (fragmentation) of solid particles is important in a range of technologies. An interesting effect is the so-called comminution limit (CL), which is effectively a brittle/ductile transition. Above the CL particles fail by fracture. However, as particle size decreases the amount of stored energy in the particle also decreases and eventually there is no longer sufficient stored energy in the particle to propagate a crack and the particle flows plastically. The CL depends on the hardness, H, and the toughness, K (Ic). In mountainous countries, two-reservoir systems are used to generate and store power. When power is needed, water runs through the turbines to the lower reservoir. If there is excess power, water is pumped to the upper reservoir. This recycling of liquid through the turbines can break up entrained particles. Previous work in this area has been primarily concerned with sedimentation of the particles. The research reported in this paper uses the CL to calculate the particle sizes produced for different materials including different rock types. Interestingly, the particle sizes predicted mainly fall in the range where they sediment near the upper water surface. In such cases, the surface layers become opaque to sunlight and plant and animal life will be affected. It is suggested that the CL provides additional information which would assist research in this area. Where H and K (Ic) are not known for a particular rock type they should be measured.