By 1940, hydropower accounted for 40% of the United States' electricity generation. This peak in hydropower's share of the U.S. energy mix marked the culmination of decades of dam-building and the rapid expansion of the electrical grid. The figure highlights the central role of water power in the nation's industrialization and the achievement of the New Deal's electrification goals, before the rise of coal and natural gas in the post-war era.

This statistic represents the high-water mark of the hydropower age in America, a moment when water was the primary source of the nation's electrical energy. This era of hydroelectric dominance is the backdrop for the industrial and social poetry of the 1930s and 1940s, a time when the promise of abundant, clean energy from water fueled both utopian visions and anxieties about the power of the state and technology.