Williston Lake (Reservoir), the largest freshwater body in British Columbia, was created in 1968 with the completion of the W. A. C. Bennet Dam. The dam blocked the flow of the Peace River and impounded the upper Peace and lower Parsnip and Finlay Rivers. Lake creation has resulted in three relatively discrete reaches: the Finlay and Parsnip Reaches lying in a north-south direction in the Rocky Mountain Trench and the Peace Reach passing through the Rocky Mountains in a west-east direction. Williston Lake waters pass through the W. A. C. Bennett Dam, Dinosaur Lake (reservoir) and the Peace Canyon Dam before entering the Peace River and ultimately the Mackenzie River system on their way to the Arctic Ocean.

Surficial geology shows Williston Lake bounded by a variety of landforms. Geological features include a series of terraces formed by glacial outwash, moraines and lacustrine deposits as well as alluvial fans and steep slopes of sedimentary rock. The major physiographic units include the Cassiar-Columbia Mountains bounding the western edge of Williston Lake, the Rocky Mountain trench of which the Finlay and Parsnip Reaches occupy and the eastern systems including the Rocky Mountains, through which the Peach Reach passes.

Historically, the river valleys were used by native tribes of nomadic hunters and gatherers. First European contact was made by Alexander MacKenzie who used the Peace River valley as his access point through the Rocky Mountains during his journey to the Pacific Ocean in 1793. Simon Fraser further opened the region establishing a series of fur trading posts in 1805. Following the decline of the fur trade came a brief period of gold exploration. The area remained the domain of prospectors, fur traders, trappers and hunters until well into the 20th century. Approval for the creation of Williston Lake in 1962 resulted in the development of pulp-sawmill facilities beginning in 1966 in the area which has become the town of MacKenzie.

W. A. C. Bennett Dam was constructed over the period of 1962 to 1967, by the B. C. Hydro and Power Authority, a provincial crown corporation in response to projected domestic power requirements. Dam construction cost was approximately $ 421 million with all associated direct and indirect costs bringing the total cost for the project to approximately $ 764 million (dollar value in year of expenditure). The dam is one of the largest earthfill structures in the world at 183 m in height and having a crest length of 2,134 m. The Gordon M. Shrum Generating Station at Bennett Dam has the largest generating capacity in the B. C. Hydro system at 2,416,000 kW.

Williston Lake has a relatively rapid flushing rate with a water residence time of approximately 2.2 years. Bathymetric characteristics reflect the lake's reservoir origin with depths progressing from river depths of 1 to 2 meters at the head of the Finlay and Parsnip Reaches to approximately 166 meters depth at the Dam (at full pool). Deeply incised former stream valleys cut through submerged benchland/terraces leading to a central channel. Submerged standing timber covers much of the lake bottom. The lake is considered dimictic and oligotrophic with excellent water quality. The characteristic low nutrient levels and low productivity is typical of most British Columbia lakes.

A $ 10-million program to enhance local fish and wildlife resources in the Williston Lake area commenced in 1988. The work is to be carried out jointly by the Provincial Ministry of Environment and B. C. Hydro using funds provided by B. C. Hydro as compensation for the creation of the reservoir (l).