During the past century, increased fertilization and the resultant algal and macrophyte growths have severely impaired the quality and use of many lakes throughout the world. A notable case has been the deterioration of Sebasticook Lake. The lake began showing signs of cultural eutrophication in the early 1950's. By the late 1960's it had become hypereutrophic and has remained in that condition despite remedial efforts. Currently a major effort to restore the lake has been undertaken by the State of Maine, U.S. Environmental Protection Agency, U.S. Department of Agriculture and local communities. Estimates of annual external phosphorus loading range from 3,900 to 11,800 kg with an estimate of 9,000 kg considered as the most accurate estimate of the current load. Sources include 2,300 kg and 2,200 kg from the towns of Dexter and Corinna respectively, and 4,500 kg from nonpoint sources of which 85 percent is culturally generated from farmland. Using a mass balance model it is estimated that an annual external load of 4,500 kg of total phosphorus would maintain the lake at a desired concentration of 15 μg/l and suppress the chronic algal blooms. Strategy to control external sources includes advanced wastewater treatment to reduce the loading from Dexter and Corinna to 300 kg and 700 kg, and improved farm practices to reduce nonpoint sources to 3,500 kg. Since the recycle of phosphorus from the lake sediments is estimated to contribute 6,900–9,900 kg into the water column annually, control of internal recycling was also imperative. Because of the large size of Sebasticook Lake (1,798 ha) the most promising means to reduce internal phosphorus was to alter the flushing regime of the lake. Epilimnetic phosphorus reaches peak concentration during late summer stratification. At that time, the lake volume is reduced by one half, decanting the phosphorus rich epilimnetic water. Refill of the lake does not commence until the following spring when phosphorus-poor melt water is available. Drawdown of the lake is accomplished through constructing a 4-meter deep canal and gate structure at the outlet. Initial trials of the structure were estimated to export about 4,200 kg of phosphorus annually.