The purpose of this paper is to investigate the association between hydroelectric energy consumption and CO2 emissions in the USA from 1980:1 to 2019:8 by using the wavelet transform model. This research revealed that (a) in the short runs (at higher frequencies), hydro energy uses intensified CO2 emissions for the periods 1990:01–1992:12, 1994:01–1994:12, and 2002:07–2007:11, and (b) during the longer periods (at lower frequencies), however, hydro energy consumption diminished CO2 emissions for the periods 1983:01–2001:12 and 2011:01–2017:03. The paper explained as well why hydro energy can yield adverse and affirmative contributions to Greenhouse gas emissions by emphasizing the role of energy generation from hydro plants in shorter runs and longer runs in the USA considering all sub-samples of the sample period 1980:1–2019:8. This research eventually suggests some energy policies to enhance the positive environmental influences of hydropower energy production in the USA.