In an upper reach of the Ishite River, the stream water is completely diverted at a weir except under flood conditions, and is regenerated below the weir by groundwater. To investigate environmental factors regulating the water quality of the regenerated stream, a 6-year monthly survey was undertaken 2.4 km below the weir between 10:00 and 14:00 from January 1992 to December 1997. Using 51 samples under base-flow conditions, a factor analysis suggested that the water quality was mainly controlled by four factors. Factor 1 accounted for 22.4% of the total variance and correlated positively with Ca2+ and Mg2+ concentrations and CB - CA (surplus strong base concentration). Since the factor 1 score correlated negatively with the flow-rate (r= - 0.618, p&lt
0.001), Ca2+ and Mg2+ concentrations seemed to be controlled by the flow-rate. Factor 2 correlated positively with Na+ and K+ concentrations and CB-CA, and was responsible for 17.4% of the total variance. Factor 3 correlated positively with SO4 2- and Cl- concentrations, and factor 4 correlated positively with pH and negatively with NO3 -, accounting for 15.4% and 13.3% of the total variance, respectively. Multiple regression analyses showed that both Ca2+ and Mg2+ concentrations were controlled by flow-rate and water temperature. The flow-rate reduced both concentrations, but the temperature increased Ca2+ concentration and reduced Mg2+ concentration, resulting in an increase in the Ca/Mg ratio.