Lake Tazawa, located in the eastern part of Akita Prefecture in Japan, has maximum depth of 423.4 m: it is the deepest lake in Japan. Temperature, pH and concentrations of chemical components of water in Lake Tazawa were analyzed during 2015–2017 to ascertain whether the lake water is circulating or not based on the lake water chemical compositions before and after neutralization measures.

　The pH values of the lake water from the lake surface to 400 m depth were 4.6–4.7 before neutralization treatment in 1989, but these values were 5.0–5.3 during 2016–2017. The pH values increased approximately 0.5 units in the lake during 1989–2017. Concentrations of SO₄^2- and Al in lake water decreased during 1989–2017. By contrast, the concentration of Ca²⁺ at all depths from the lake surface to 400 m depth increased during 1989–2017. The increase of the Ca²⁺ concentration was affected by increased Ca²⁺ concentrations in river water of the Tama River, which flows into Lake Tazawa. The neutralization measures used limestone (CaCO₃) in the Tama River upstream. The increased Ca²⁺ concentration in Lake Tazawa waters at all depths reflects the presence of vertical water mixing. Results show that improvement of Lake Tazawa water quality is possible because of water mixing that occurs with introduction of river water that has proper water quality.

　Stream water in headwater catchments is useful as a source of water for domestic use during long-term water outages caused by disasters in mountainous areas. Therefore, evaluating its availability characteristics and stream water quality as a water source is necessary for effective water use. We investigated 59 watersheds covered by Mesozoic sedimentary rock (27 ha average watershed area) on the edge of the Nobi Plain in western Gifu prefecture, Japan. Discharge of the streams was 17–9,361 m³/day. Discharge of springs was 2–432 m³/day. Headwater catchments with drainage area of ≥40 ha and relief height of ≥500 m tended to have high specific discharge with slight variation. Moreover, they exceeded the national average of ecological maintenance flow (0.73 mm/day or 8–2,574 m³/day). Water quality analysis and the stable isotope ratio suggest that catchments exceeding the ecological maintenance flow are stable water sources because of the dominant contribution of bedrock groundwater. Understanding hydrological processes in mountainous watersheds is useful not only to secure water for domestic use during disasters, but also as basic information for regional sustainable water resource management.

　Since the founding of Nagoya, the Horikawa River has been a symbol of the region. Improving the water quality of this artificial river with no self-flow has persisted as a challenge since the Meiji Era, when urbanization and industrialization rose to prominence. Among various methods, dilution by increasing the flow rate has been the most used method, with water quality improvement effectiveness confirmed recently from a three-year "social experiment" that began in 2007, followed by two-year observations. The outcomes of that effort are being inherited at the citizen level even now. As described herein, based on the continuity from the Kiso River, we examined methods to secure the flow rates in both the non-irrigated and irrigated periods using existing agricultural facilities of local resources, applying the 0.4 m³/s flow rate at the time of the "social experiment" as a benchmark.

　The flow from the Kiso River to the Horikawa River has long been used for irrigation via the Kottsu-yōsui and Shin-kottsu-yōsui, with an accompanying history of use for navigation. Although this trial proposal has some potential for use as a practical example of the 'collaboration' of stakeholders in basin flood control, Ryūiki-chisui, it was also found to entail various challenges.