Miyun Reservoir, located 100 km northeast of Beijing City, is a mountain valley reservoir. It was built in September,1960, and is the largest reservoir in Beijing area. Two main rivers, Chaohe and Baihe, flow into Miyun Reservoir. The catchment is about 15,788 km2, consists of mountains and piedmonts and lacks large industrial enterprises. Sediments and nutrients from agricultural, pastoral and forestry lands that enter the reservoir with overland flow make the main pollution sources.

The initial purpose of building Miyun Reservoir was flood control, irrigation and fishery, but it has become more and more important as a main drinking water storage for Beijing area. The water quality is therefore of great importance.

Recently, the nutrient concentration and the number of phytoplankton in the Reservoir are constantly rising to arrest the attention of Beijing municipality. A project for keeping the water clean is being put into effect (Q).