Data of snapshot sampling in Ishikari River basin

The six study sites lie within the Ishikari River basin in Hokkaido, northern Japan. The forested mountain area of the Ishikari basin can be divided into two main parts depending on geology: the Yubari Mountain Range, which is underlain by sedimentary bedrock, and the eastern part of the basin, which is underlain by igneous rock formed by the activity of volcanic groups, including the Daisetsuzan and Mt. Tokachi. The S-SE, S-NA, and S-PO study sites were located on the Yubari Mountain Range and underlain by sedimentary bedrock, and the V-IS, V-PE, and V-KU study sites were located around the Daisetsuzan volcanic group and underlain by volcanic bedrock.Spring and stream water were sampled for chemical analysis and discharge rates were measured during baseflow conditions at points with different catchment areas.Streamflow velocity was measured by a propeller flow velocity meter (CR-11; Miura-Rika Inc., Sapporo, Japan). Discharge was calculated by multiplying the cross-sectional area by the stream velocity. Specific discharge was calculated as the measured discharge rate divided by the catchment area obtained from the Geospatial Information Authority of Japan’s 10 × 10-m digital elevation models using QGIS 3.4 software (QGIS Development Team).The samples were filtered through a GF/F filter with 0.63-µm pores in a laboratory. The samples were sealed in polyethylene bottles and refrigerated at 2°C until analysis. The solute concentrations were analyzed in a laboratory. Anion and cation concentrations were measured using ion chromatography (Dionex ICS-1100; Thermo Fisher Scientific, Waltham, MA, USA), and SiO2 concentrations were measured using an auto-analyzer (QuAAtro 2-HR; BLTEC, Osaka, Japan).