As California's State Engineer, Hall worked on a comprehensive water supply and flood control system for the Sacramento Valley. Hall's study of California's hydrology lasted from 1878 through 1883. In that time, his staff installed an extensive flow gauging system along some of California rivers. He was also instrumental in designing projects to help San Francisco acquire adequate supplies of water from the western watershed of the Tuolumne River. Following the earthquake of 1906, San Francisco was able to secure the rights to the water, and it flooded Hetch Hetchy Valley.