Jack Lake is just south of Chandos Lake and east of Apsley. It is a good size and deep lake, with many islands, embayments, and an irregular shoreline. It is within the Peterborough Crown Game Preserve, and a considerable portion of its shoreline is crown land. It is part of the Trent-Severn system and drains into Stony Lake. As such, it is a water reservoir for the Trent Canal system, and its level is controlled by a dam which can vary the level by as much as 5 feet, based on the dam min/max, but as seen in the graph below, the yearly variation is more like 2.5 feet.
Jack Lake spans 2 different townships, and has a natural environment that transitions from Southern Ontario to the Canadian Shield, providing a highly varied flora/fauna/geology.