Lake Lillinonah, a hydroelectric impoundment on the Housatonic River, CT, is characterized by frequent and extensive algal blooms during the summer months, and historical accounts from the lake's early years document the fact that algal blooms (dominated by cyanobacteria) have been a concern since the lake's creation in 1955. Algal blooms create lethal oxygen conditions for aquatic organisms, impair recreation, and produce toxins that are harmful to people, pets, and wildlife. To help understand current and future trends in water quality, we reconstructed the historical water quality of Lake Lillinonah from 1974 to 2009. Our results suggest that water temperature, phosphorus concentration, and nitrogen concentration all play a role in determining summer water clarity. Additionally, although total phosphorus concentration has decreased since the early 1970s, total nitrogen concentration has remained constant likely due to differences in watershed nutrient-management strategies, and water clarity in the lake remains poor. We suggest that a continued effort to reduce both nitrogen- and phosphorus-loading is necessary in order to improve water clarity, particularly considering the observed increase in storm-loading events and warmer temperatures predicted as the climate warms.