Lake Reisenji is an artificial reservoir in northern Nagano Prefecture in the shadow of the volcanic Mount Iizuna—a part of the Myoko-Togakushi Renzan National Park.
Swimming in the lake is not permitted, but of course there are swan shaped pedal boats and row boats for pottering around. The lake is also stocked with carp and black bass for fishing. Based on the number that break the surface of the lake, they are quite plentiful.
In addition to the on water activities, there is a hiking course around the lake, mountain biking, golf, tennis, and an onsen. There is public parking near the Tengu no Yakata onsen at the southeast corner of the lake. You can also ask about fishing rules and fees at Tengu no Yakata.
Iizuna Higashikogen Camping Ground is also located within a short walk to the lake.