The water sector is facing significant challenges worldwide as a result of climate change. Drinking water utilities need to incorporate climate considerations into long-term planning and management; they need to become climate ready. Aging water infrastructure, high costs for maintenance and treatment, increasingly stringent environmental regulations, and inadequate funding are all obstacles to achieving water utilities' goals. Two effective features of a sustainable drinking water supply are water storage and elevation. Water storage enhances water supply sustainability, and elevation enables the earth's gravity to drive water and produce hydroelectric power naturally and inexpensively. This article reviews a case study of a successful century-old water system characterized by a large reservoir for storing high-quality mountain water and by gravity-driven water transmission; the focus is on the Hetch Hetchy Regional Water System in California, with a comparison with similar reservoirs in the United States and Europe.