Xi-hu (West Lake) is a small freshwater lake situated close to the city of Hangzhou in the southeastern coastal area of China. The scenery of the lake has been famous for its picturesque interfusion of hills and water since the age of Tang Dynasty (618-907 A.D.). The lake originated from an estuary and evolved into the present-day freshwater lake through ages. According to historical records, dredging had been conducted in the lake more than 23 times; thus human activities played an important role in the formation and protection of the lake.

It is divided into five sections by artificial causeways. The Outer (Wai) Lake is the main body which accounts for 82.4% of the total lake volume. The annual inflow of natural runoff water into the lake from its small drainage basin is only 1.5 times the lake volume, so the retention time of lake water is relatively long.

The drainage basin consists mainly of eroded limestone hills. Being favored by a subtropical monsoon climate with abundant rainfall, up to 70% of the basin area is covered by forests of mixed evergreen and deciduous broadleaf trees and conifers. A spell of hot sunny days prevails through July and August, and the average winter temperature is well above the freezing point.

Tourism makes the largest contribution to the basin's economy. The number of tourists visiting Xi-hu amounts to 7 millions in recent years. Traditional tea plantation is the leading type of local agriculture. Eutrophication is a serious threat to the lake, although the rainwater and wastewater from the urban area of Hangzhou do not enter the lake. A series of such control and remedial measures have been taken by responsible authorities as the interception of inflowing wastewater, diversion of clean water from the Qiantang River into the lake through a canal, control of point sources of pollution, dredging of sediments, construction of protection bank, use of electric leisure boats, controlled fishery, etc. (1).