Mortality of brook trout Salvelinus fontinalis and water chemistry were characterized in 6 headwater streams in the western Adirondacks of New York during spring 2015, 2016, and 2017 and compared with results from analogous tests done between 1980 and 2003 in many of the same streams, to assess temporal changes in toxicity and inorganic monomeric aluminum (Al_i) concentrations, and the role of Al_i exposure duration on brook trout survival. The Al_i concentrations of 2 and 4 µmol L^–1 corresponded to low‐to‐moderate and high mortality thresholds, but prolonged exposure to ≥1 µmol Al_i L^–1 also produced mortality. The variability, mean, and highest Al_i concentrations in Buck Creek year round, and in several other streams during spring, have decreased significantly over the past 3 decades. Logistic models indicate that Al_i surpassed highly toxic concentrations in Buck Creek for 3 to 4 mo annually during 2001 to 2003 and for 2 to 3 wk annually during 2015 to 2017. The loss of extremely high Al_i episodes indicates that toxicity has declined markedly between the 1989 to 1990, 2001 to 2003, and 2015 to 2017 test periods, yet Al_i concentrations can still cause moderate‐to‐high and complete (100%) mortality. The logistic models illustrate how mortality of brook trout in several Adirondack streams likely decreased in response to the 1990 Amendments to the United States' Clean Air Act (which decreased acidity, Al_i concentrations, and duration of toxic episodes) and offer a means to predict how changes in US regulations that limit emissions of NO_x and SO_x (and N and S deposition loads) could affect fish survival and stream ecosystems in this region and across the Northeast.