<p>　In this study, natural calamities arising from the Mt. Ontake-san volcano and fluctuations in the pH values in the Ootaki-gawa River system, Nagano Prefecture, Central Japan, are investigated based on various reports by researchers, local governments, residents, and anglers. During the last 50 years, Mt. Ontake-san erupted twice, in 1979 and 2014, and an earthquake in 1984 caused a large-scale landslide. These events may have affected local river water quality, aquatic biota, and the livelihood of residents who depended on the local environment. However, the causes and effects of each of the above events have not been sufficiently analyzed; previously, volcanic ash fall was regarded as the only cause of the ensuing problems. The mechanisms of pH change varied for different events, and for the different types of aquatic environments affected by these events.<br>　Following the 1979 and 2014 eruptions, field observations from the Nigori-gawa River, which has its origin in Mt. Ontake-san, exhibited no direct evidence of a decrease in the pH value of its waters due to the volcanic ash fall. In these cases, the decline in the pH values of the Nigori-gawa River may have resulted from an increase in both the discharge flow rates and sulfuric acid concentrations in ground water. These changes were also observed after the 1984 earthquake, a geological event that was not accompanied by an ash fall.<br>　The landslide caused by the earthquake in 1984 buried the upper Nigori-gawa and the Denjo-gawa, the two biggest tributaries of the Nigori-gawa River, and the pH values in the lower Nigori-gawa rose, even though the sulfuric acid concentrations in the river water increased compared to the levels before the landslide. Some events, such as the increase in the activity of aquatic primary producers and the use of concrete for anti-erosion restoration, may have acted as neutralization mechanisms for the pH level.<br>　The pH values in the Makio-dam impoundment, into which the Nigori-gawa discharges, dropped to a lower value after the eruptions in 1979 and 2014. The decline did not result from the direct effects of the ash fall but from microbe-mediated production of sulfuric acid from pyrite (FeS₂), which is found in the ash deposited at the bottom of the impoundment. Stable low pH conditions were probably established by repeated supply of ash-containing pyrite into the impoundment, by heavy rainfall events in the catchment area.<br>　These two eruptions had only a few direct economic implications for the local community, and the decrease in population of the Ootaki-mura Village after the events was not significant. Instead, the population had started to decrease from the early 1950s, mainly due to the construction of the Makio-dam impoundment, which resulted in the productive fields of the village being immersed. The decline in the population of Ootaki-mura is only one of many examples found in similar dammed rivers in Japan, and this was not found to be linked to the natural calamities arising from the volcanic eruptions.</p>