Lake Kapowsin is a lake in Pierce County, Washington, about halfway between Tacoma on Puget Sound, and Mount Rainier in the Cascade Mountains. The lake is 2.5 miles (4.0 km) long and 0.15–0.5 miles (0.24–0.80 km) wide, lying in a channel formed by meltwater from the Puget lobe of the Vashon glacier during the Pleistocene glaciation. A small island, Jaybird Island, lies in the northern half of the lake. As indicated by a drowned forest in the lake and other evidence, the Puyallup River was inundated about 550 years ago by a lahar from Mount Rainier called the Electron Mudflow. The mudflow partially filled the channel and blocked Ohop Creek's outlet, forming present-day Lake Kapowsin.