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Abstract
This paper evaluates large-scale hydroelectric dams built in the United States during the twentieth century. Combining panel event-study designs and synthetic control methods, two results stand out. First, dams constructed pre-1950 spurred short-run local growth, in large part thanks to a 'cheap-local-power advantage', and resulted in remarkable long-run growth, more than doubling local population density after 50 years. Second, dams constructed post-1950 had only modest effects on growth. The first result indicates agglomerative impacts on local economic activity. The second result suggests that the cheap-local-power advantage created by hydropower attenuated after 1950, probably because of such innovations as high-tension transmission lines.

The 1918 Influenza Pandemic killed millions worldwide and hundreds of thousands in the United States. This article studies the impact of air pollution on pandemic mortality. The analysis combines a panel dataset on infant and all-age mortality with a novel measure of air pollution based on the burning of coal in a large sample of U.S. cities. We estimate that air pollution contributed significantly to pandemic mortality. Cities that used more coal experienced tens of thousands of excess deaths in 1918 relative to cities that used less coal with similar pre-pandemic socioeconomic conditions and baseline health. Factors related to poverty, public health, and the timing of onset also affected pandemic mortality. The findings support recent medical evidence on the link between air pollution and influenza infection, and suggest that poor air quality was an important cause of mortality during the pandemic.