The West Lake of Hangzhou, a famous scenic spot for tourist, has become a nutritive lake with a turbid water body and unchecked proliferation of algae, which has seriously damaged the attractiveness of its beauty. The Lake, with an area of 5.6 square kilometres and an average depthof 1.55 metres, is artificially controlled in its water level, whereas the current velocity of the Lake may exceed 0.1m/sec. during flood water period, but it turns a "dead lake" during dry season. The Lake has a regular current direction but meagre sources of water, the water body was renewed for 1.2 times during the dry year of 1979. The temperatures of water vary very little both horizontally and vertically. The transparency of the Lake water has reduced from 0.5 to 0.35m,while the pH value risen from 7.1-9.2 to 8.1-10.2 over thirty years. In recent years, in consequence of 400 tons of sewerage drained daily into the Lake, the ammonia nitrogen rises to 0.04-12.0mg/l, the average value of total nitrogen to 2.76 ppm and that of total phophorus to 0.13ppm. The Lake, containing 60,000 algae, primarily blue and green algae, each millimetre of water, has become eutrophic. The unfavourable hydrological conditions of environment and the large quantities of sewerage drained into the Lake account primarity for eutrophication of the water body. To draw water from outside of the valley as an additional source so as to quicken water circulation in the Lake is one of important measures for improvement of the hydrological conditions and the cure of eutrophication of the West Lake.