We present Global Positioning System (GPS) measurements of crustal deformation caused by seasonal filling and emptying of four surface reservoirs of varying dimensions located in different geological domains of India. We find that all the reservoirs, including the smallest Dharoi reservoir with a storage capacity of only , cause deformation in the neighboring region. Such deformation leads to stress change, and depending on the availability of critically stressed faults in the region, a reservoir may trigger earthquakes that can be detected if a local seismological network is in place. Interestingly, three of the reservoirs, the Tehri, Koyna, and Ukai reservoirs, triggered earthquakes in their vicinity. Thus, we suggest that there appears to be no lower threshold on the size of reservoir to cause deformation in the surrounding medium.