Due to recent urban expansion, a trend has developed wherein cities are seeking sources of municipal water beyond the divide of the river basin. Such diversions of river flows impact aquatic environments in several ways. When considering current and future problems with water resources and water pollution, it is extremely important to clarify the influence of river diversions for municipal water supply on the aquatic environment. Such interbasin water transfers for municipal supply are notably seen in the Tokyo metropolitan area, Kanagawa prefecture, Aichi prefecture, and Kagawa prefecture; this paper focuses on the historical development of water resources in Kanagawa prefecture (Fig. 1). Large-scale water diversions have been made using aqueducts to transport water from rivers in Kanagawa prefecture to Yokohama, Kawasaki, Yokosuka, and other cities near Tokyo, resulting in increased discharges into Tokyo Bay and decreased discharges into Sagami Bay.<br> The purpose of this paper is to elucidate these changes in the amounts of water diverted for city use and shed light on the restoration of municipal water and sewerage systems. The total volume of water diverted from the Sagami and Sakawa Rivers for municipal use from fiscal year (FY) 1960 to FY 1990 has been calculated. The data were processed using GIS, and the results are shown in Figs. 2-7.<br> 1) The volume of total intake water has been increasing annually since FY 1960. This amount was 480 million m³ in FY 1960, and increased to 1, 750 million m³ in FY 1990 (Fig. 2).<br> 2) The total volume of treated wastewater was 120 million m³ in FY 1960, and increased to 950 million m³ in FY 1990 (Fig. 3).<br> 3) The total volume of water intake from the Sagami River basin was 360 million m³ in FY 1960, increasing to 1, 055 million m³ in FY 1990 (Fig. 4).<br> 4) The total volume of treated wastewater, which was originally intake from the Sagami River basin, was 70 million m³ in FY 1960, increasing to 700 million m³ in FY 1990 (Fig. 5).<br> 5) The total volume of water intake from the Sakawa River basin was 180 million m³ in FY 1960, increasing to 280 million m³ in FY 1990 (Fig. 6).<br> 6) The total volume of treated wastewater, which was originally intake from the Sakawa River basin was 50 million m³ in FY 1960, increasing to 160 million m³ in FY 1990 (Fig. 7).<br> 7) The volume of water diverted from the Sagami River basin to Tokyo Bay basin has been increasing in recent years. The volume of water used was 200 million m³ in FY 1960, rising to 568 million m³ in FY 1990. With the increase in new and improvement of existing sewer connections, the discharge of treated wastewater into Tokyo Bay has also increased. The volume of treated water was 68 million m³ in FY 1960, increasing to 496 million m³ in FY 1990 (Fig. 9).<br> It is presumed that the environment of the lower reaches of the Sagami River has been influenced by decreasing discharges due to these diversions.