An Instant inland sea
Qiandao Lake, or Thousand Island Lake, is a man-made reservoir formed in 1959 by the construction of the Xin'an River Dam for hydroelectric power. The dam, standing 105 meters (344 feet) high, inundated a valley that contained the ancient cities of Shi Cheng (Lion City) and He Cheng. The flooding created a vast body of water with a surface area of 573 square kilometers and a storage capacity of 17.8 cubic kilometers. This event submerged over 1,300 villages and displaced approximately 300,000 people. The result is an area of 1,078 islands, which are the former hilltops of the flooded valley. The lake has an average depth of 34 meters and a maximum depth of over 100 meters, allowing for complex limnological processes.
A stratified laboratory
As a deep subtropical reservoir, Qiandao Lake exhibits a distinct monomictic thermal stratification, meaning it mixes once a year. A stable stratification period lasts from approximately April to December each year. During this time, the lake separates into three layers: a warm, oxygenated upper layer (epilimnion), a middle layer with a rapid temperature drop (thermocline or metalimnion), and a cold, oxygen-poor bottom layer (hypolimnion). Surface water temperatures can range from 10.4°C to 32.7°C throughout the year.
This stratification directly influences the lake's water quality and biology. Dissolved oxygen levels become very low in the bottom layer during stratification, a state known as hypoxia. This condition affects nutrient cycling, particularly the release of phosphorus and nitrogen from sediments, which can fuel algal growth in the upper layers. Studies have identified a subsurface chlorophyll maximum, where phytoplankton thrive at depths of 1.5 to 5 meters below the surface. The lake hosts more than 100 fish species, and their distribution is heavily influenced by these temperature and oxygen gradients. For example, dominant species like silver and bighead carp tend to occupy the upper 10 meters of the water column. The water quality is generally good, transitioning from a mid-mesotrophic state, and is clean enough to be the source for the popular Nongfu Spring bottled water brand.