As described in the Wikipedia page, the power load balancing is done over the course of a day - not seasonal (those tend to be reservoirs for consumption / irrigation).

> Along with energy management, pumped storage systems help control electrical network frequency and provide reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand, potentially causing frequency and voltage instability. Pumped storage plants, like other hydroelectric plants, can respond to load changes within seconds.