Jack Lake is a small headwater/ feedwater lake in Ontario, Canada, within the northern portion of the Kawartha lakes region which is located at the southern end of the Canadian Shield. The clean freshwater lake which has a reasonably complex limnology is located in northern Peterborough County, approximately 100 mi (160 km) directly northeast of Toronto. There is a resort on the north-western shore of the lake, and the nearest town is Apsley, Ontario.
The lake can be generally characterised as having a fairly irregular shape, dominated by 3 deeply indented main bays and having numerous smaller or tertiary bays. The largest bay is Sharpe Bay on the western end of the lake. The lake has served as a field research centre, water quality monitoring site and testing or analysis base for Environment Canada's National Water Research Institute (NWRI) through into the 1990s. Specific and longitudinal research work has been supported by a variety of world class academic centres and Universities over the past few decades. Work done during this period has helped to improve understanding of how freshwater environments are impacted upon by development, recreation and wider changes within the natural environment.
Jack Lake has a long tradition of welcoming anglers from around the world. It was originally home to bass and trout. As the lake developed as a tourist attraction in its own right, and as a feeder lake within the wider catchment that supports the Trent-Severn Waterway, fish species such as muskellunge, walleye, rock bass, and perch were also introduced from approximately the 1920s. More recently, Black Crappie have been introduced and are a primary target for summer and winter fisherman. Ongoing provincial and federally supported environmental stewardship and related work experience initiatives occur on the lake from year to year.