Water level of Lake Kawaguchi in Yamanashi Prefecture had risen more than 3 m above the standard level after the heavy rainfalls in summer of 1983. On that opportunity the authors started to study the hydrogeological conditions around the lake with the use of long-term observation data of the lake water level, groundwater level, precipitation and others (Figs.4 and 5).
The area is composed mainly of the Tertiary Misaka Group and the Quaternary volcanic rocks extruded from Fuji Volcano (Fig.1). The Misaka Group and the Kofuji Mud-flow Deposits, one of the effusive rocks of the volcano, constitute the hydrogeological impermeable bed rocks.
Although Lake Kawaguchi has no natural mouth for surface discharge, the water is drained off through the man-made tunnels. On the other hand, it is likely that the water permeates through the volcanic rocks into the underground valley about 5 km south of the lake, judging from the contour line of the groundwater level (Fig.2). Figures 2 and 3 indicate that the hydrogeological watershed of the lake occupies only the northern part of the topographic watershed which extends southwards to the top of Mt. Fuji.
The lake water is recharged from the surrounding mountains of the Misaka Group. The lake water remarkably rises after the 3 days rainfall reaches more than 200 mm (Figs.6 and 7). When the 3 days rainfall is less than 100 mm, it is invisible. Fluctuations of the lake water coincide with the rainfall pattern (Fig.4). The graph of the accumulation value of the rainfall variation is similar to the fluctuation of the lake water level.