The reservoirs will at some point see noticeable sediment buildup, but not at all comparable with surface pumped hydro based on blocking valleys, due to the cyclic nature of the water flow in the hydrostor facility. And occasional cleanout (measured in decades or centuries?) will be trivially cheap compared to construction cost. Very much unlike cleanout cost behind valley dams, which are would-be cleanout costs because that never ever happens as it would utterly dwarf the cost of the original dam. There's been an article linked here a few months ago (can't find it, unfortunately) about how the total capacity of pumped hydro is getting increasingly smaller each year, despite new sites getting built. This is because sedimentation already outpaces the buildup, and it will only get worse the more we build. The volumes accumulating behind a dam are just too big, unless you have zero natural flow from rainwater (and then just getting the working volume of water to the site would be prohibitively expensive, per capacity, even before you factor in evaporation - the cycle capacity per unit of water is just so much lower than what a hydrostor site would achieve with its much larger head plus the energy stored in air compressionand heat)