It was determined by oxygen isotopic and chemical composition analysis that most of the rainfall which fell in the upper stream of the Ishi river basin did not run off directly but rather ran off after it had infiltrated into the ground. Therefore, in the Ishi river basin upper stream, no remarkable change in water quality occurred even despite fluctuations in rainfall amounts. However water quality was changed by types of land use. The sulfur of SO₄^2- found in the upper stream water was estimated by sulfur isotopic ratios to have mainly come from the Izumi formation. The Ca²⁺ and HCO₃^- found in the upper stream water were thought to have come from CO₂ gas and feldspar contained within the soil of the forests during weathering. Ca²⁺, HCO₃^-, NO₃^- and SO₄^2- of river water in orchards and rice fields were thought to have come from fertilizer. The Na⁺, Cl^-, Ca²⁺, HCO₃^-, NO₃^- and SO₄^2- found in the river water of housing sites were thought to have derived from sewage.