The oldest method of hydroelectric power was using river current. It was still common in the early 21st century.
The waterwheel was invented in ancient Greece. It was used for irrigation and as a power source. The technology spread quickly. Even during the Middle Ages, the waterwheel was very important. As the Industrial Revolution began, the waterwheel started to be used to power factories. As time went on, the waterwheel was replaced with the water turbine. The water turbine became one of the main components of the hydroelectric dam.
Tech Level: 8-10
The first use of dams for hydroelectricity was in 1882. The water went into the dam and turned a water turbine which, in turn, turned a generator. This created electricity. As dams got larger like Hoover Dam, they started being used for flood control, irrigation, and even navigation. Authorities such as the Tennessee Valley Authority were created to build dams during the Great Depression. Dams continued to be used into the 21st century. By the 2020s, hydroelectric dams were even being used as arcologies with all the dam's non-support structure used as living and arcology space. Meanwhile, a new source power was being used: Tidal Power.